What contributes to flight ace being held in prison (5)
I believe the answer is:
stair
'what contributes to flight' is the definition.
(as in a flight of stairs)
'ace being held in prison' is the wordplay.
'ace' becomes 'a' (abbreviation).
'being held in' indicates putting letters inside.
'prison' becomes 'stir' (informal term for prison).
'a' put within 'stir' is 'STAIR'.
(Other definitions for stair that I've seen before include "Rising step" , "Word linking way and well" , "single step in a series" , "Step as part of flight" , "One of a 15" .)