Careless agent retaining cut (6)
I believe the answer is:
sloppy
'careless' is the definition.
(similar in meaning)
'agent retaining cut' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'retaining' is an insertion indicator (inserted letters are held or retained).
'cut' becomes 'lop' (lopping is a kind of cutting).
'spy' placed around 'lop' is 'SLOPPY'.
(Other definitions for sloppy that I've seen before include "Slipshod" , "Romantic" , "Excessively casual" , "Runny" , "Careless and messy" .)