Declaration: 'It's a career' (10)
I believe the answer is:
profession
This clue is a double definition.
'declaration' is the first definition.
(I know that profession is a type of affirmation)
'a career' is the second definition.
(I know that career can be written as profession)
'it's' acts as a link.
(Other definitions for profession that I've seen before include "statement" , "Job" , "career?" , "Occupation; declaration (of belief)" , "announcement" .)