A small bird, grey (5)
I believe the answer is:
ashen
'grey' is the definition.
(I've seen this before)
'a small bird' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'bird' becomes 'hen' (hen is a kind of bird).
'a'+'s'+'hen'='ASHEN'
(Other definitions for ashen that I've seen before include "Like bird" , "Livid" , "Pale-faced" , "Pale in complexion" , "Anaemic-looking" .)