Question troubles birds (6)
I believe the answer is:
quails
'birds' is the definition.
(quail is a kind of bird)
'question troubles' is the wordplay.
'question' becomes 'qu'.
'troubles' becomes 'ails' (as verb).
'qu'+'ails'='QUAILS'
(Other definitions for quails that I've seen before include "Small game birds" , "Game birds - shows fear" , "Flinches" , "Cringes before the game birds" .)