Amateurish hack covered up by agent (6)
I believe the answer is:
sloppy
'amateurish' is the definition.
(thesaurus)
'hack covered up by agent' is the wordplay.
'hack' becomes 'lop' (both can mean to cut up).
'covered up by' means one lot of letters goes inside another.
'agent' becomes 'spy' (spy is a kind of agent).
'lop' going inside 'spy' is 'SLOPPY'.
(Other definitions for sloppy that I've seen before include "Slipshod" , "Romantic" , "Casual; sentimental" , "maudlin" , "Mawkish" .)