Like all poetry, meaning is left to the reader. Poetry is subjective- what this means is that a reader determines meaning through the emotion they bring to the reading. This meaning is based off of a reader's knowledge of a word, understanding of a word, and their own feelings about the linking of words.
Many lines in poetry are very hard to define when taken out of context. Too many defining understanding lie within one word,or groups of words.
This being said, one could interpret the following line "fair creature of an hour" (from Keats' "When I have fears") in many different ways, as you have stated in your question.
What needs to be done is justification of your answer. If you believe that he is refering to poetic inspiration, you would justify this statement by providing support using the following lines:
Before my pen has glean'd my teeming brain,/ Before high pil'd books, in charact'ry
If you are supporting the defining of the line as one which focuses on spiritual being, then you would use the following lines to support:
Never have relish in the faery power
If you are looking to support the line referring to ones love, then you would support it by using the following line:
Of unreflecting love
Regardless of which you choose, you must make sure to support your answer using text from the poem itself.
I would tend to agree with the stand that the line refers to a love interest given that in the final line Love is spoken of in regards to one sinking without it.
The first link below offers another point-of-view as given by the eNotes site. Check the link out for another perspective.