Keen to eat old duck (5)
I believe the answer is:
avoid
'duck' is the definition.
(thesaurus)
'keen to eat old' is the wordplay.
'keen' becomes 'avid' (I've seen this in other clues).
'to eat' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'avid' placed around 'o' is 'AVOID'.
(Other definitions for avoid that I've seen before include "Miss" , "Shy away from" , "Shun, dodge" , "Eschew" , "invalidate" .)