A small fowl, grey (5)
I believe the answer is:
ashen
'grey' is the definition.
(I know that ashen is a type of grey)
'a small fowl' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'fowl' becomes 'hen' (hen is a kind of fowl).
'a'+'s'+'hen'='ASHEN'
(Other definitions for ashen that I've seen before include "(Of face) pale (with shock)" , "Drained of colour" , "Grey, pallid" , "Pale of face, quite grey" , "of Tree" .)