Cake in bed, unfinished (3)
I believe the answer is:
bun
'cake' is the definition.
(thesaurus)
'bed unfinished' is the wordplay.
'bed' becomes 'bunk' (bunk is a kind of bed).
'unfinished' means to remove the last letter.
'bunk' with its final letter removed is 'BUN'.
'in' acts as a link.
(Other definitions for bun that I've seen before include "Tied hair" , "Hair in tight coil" , "One from Chelsea" , "Kind of small round cake" , "Small cake; hair style" .)