In Ch.41, Pip takes Magwitch to a safe place in Essex street and then returns to his apartment. Once alone with Herbert, Pip tells him that he cannot take any more money from Magwitch and that he would like to immediately disassociate himself from Magwitch. Pip then remarks ruefully, "I am heavily in debt, and I have been bred to no calling, and I am fit for nothing." Herbert tries to comfort him but Pip says that all he is fit for and is capable of becoming is a soldier.
In those days, only those who were utterly desperate and have completely lost all hope in their future prosects will decide to join the army. Herbert comforts him by saying "soldiering won't do." He suggests very ironically that Pip join his firm 'Clarriker's.'
in Ch.58 Pip tells us that's exactly what he did, "within two months I was clerk to Clarriker and Co."