Does Melvin Burgess have an overt and/or a hidden agenda he is trying to express in Smack?
Let's clarify the language of what you are asking so you know precisely what you want to find out about Burgess. Agenda is a list of matters to be taken up or of actions to be considered or undertaken at a gathering such as a meeting (Collins Dictionary). By definition, an agenda is always "overt" and public and clearly represented [overt: open to public view; observable by participants and onlookers (Collins)]. Thus saying "overt agenda" is a misleading redundancy.
If "overt" is the part that is of interest, then it is correct to say "overt objective" [objective: relating to a private or public goal worked toward (Collins)]. If "agenda" (public observable list of tasks to do or consider) is the part that interests, then it is correct to simply say "agenda": Does he have an agenda in this novel?
On the other hand, a common English idiom is the phrase "hidden agenda." While by definition there is no parallel to this in "overt agenda" (as explained), a hidden agenda is a secret plan or intention; a hidden motive or ploy; an ulterior motive (Collins). Thus while Burgess may have an overt objective (as objectives and aims may be secret or observable), he may also have a hidden agenda known only to himself.
Does Burgess have an overt objective?: Yes. His overt objective or overt intention is to tell the story--without judgement or disparagement or pity--of the street teens who live on the streets of or take squatters rights in England's large towns. He tells the story through the first person voices of his characters to eliminate censorious judgement, condemnation or pity.
Does Burgess have a hidden agenda?: This one is a little harder to answer as hidden agendas in narratives can only be inferred from textual or biographical evidence. Using Burgess's biography, we know he lived on the streets and as a squatter in England for the better part of three decades, the 1960s, 70s and into the 80s. We know he and his brother had the same shocking experiences that are witnessed of in Smack. We know that in the text, rather than use a judgement passing, action and thought censoring third-person narrator, Burgess chose to let the characters speak on their behalves from their own perspectives without the censorship of a judging or pitying consciousness.
From this textual and biographical evidence, it seems fair to say that Burgess does have a hidden agenda and that it might be to devillainize and devictimize youths living on the streets; it might be to show their humanity and similarity over and against their peculiar differences, especially since he and his brother were themselves once among the street dwellers and squatters.