One incarcerated in stir for bird (6)
I believe the answer is:
budgie
'for bird' is the definition.
(budgie is a kind of bird)
'one incarcerated in stir' is the wordplay.
'one' becomes 'i' (Roman numeral).
'incarcerated in' means one lot of letters goes inside another.
'stir' becomes 'budge' (synonyms).
'i' put within 'budge' is 'BUDGIE'.
(Other definitions for budgie that I've seen before include "Small bird, domestic favourite" , "Common Australian cage-bird" , "Pet bird (abbr)" , "Small colourful cage bird, in brief" , "Cage-bird from Australia, in brief" .)