Pale when chicken (5)
I believe the answer is:
ashen
'pale' is the definition.
(synonyms)
'when chicken' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'chicken' becomes 'hen' (hen is a kind of chicken).
'as'+'hen'='ASHEN'
(Other definitions for ashen that I've seen before include "(Of face) pale (with shock)" , "Greyish" , "Visibly shocked" , "of Tree" , "Grey-faced" .)