Careless agent, cut off internally (6)
I believe the answer is:
sloppy
'careless' is the definition.
(synonyms)
'agent cut off internally' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'cut off' becomes 'lop' (I've seen this in another clue).
'internally' is an insertion indicator.
'spy' going around 'lop' is 'SLOPPY'.
(Other definitions for sloppy that I've seen before include "Lacking neatness and order" , "Runny" , "Imprecise; wet" , "Careless and messy" , "Mawkish" .)