Portion including a carrot (4)
I believe the answer is:
bait
'carrot' is the definition.
(thesaurus)
'portion including a' is the wordplay.
'portion' becomes 'bit' (both can mean a part).
'including' means one lot of letters goes inside another.
'bit' enclosing 'a' is 'BAIT'.
(Other definitions for bait that I've seen before include "Used to lure fish or other animals into danger" , "Set a trap with food" , "Food used as lure" , "Enticement, lure" , "Taunt; enticement" .)