Work at length to stop secret agent becoming careless (6)
I believe the answer is:
sloppy
'careless' is the definition.
(I've seen this before)
'work at length to stop secret agent' is the wordplay.
'work' becomes 'op' (abbreviation for opus**).
'at' says to put letters next to each other (I've seen this in other clues).
'length' becomes 'l'.
'to stop' means one lot of letters goes inside another.
'secret agent' becomes 'spy' (spy is a kind of secret agent).
'op' after 'l' is 'lop'.
'lop' placed into 'spy' is 'SLOPPY'.
'becoming' acts as a link.
(Other definitions for sloppy that I've seen before include "Casual; sentimental" , "Mawkish" , "Romantic" , "Imprecise; wet" , "maudlin" .)