Mawkish agent without work (5)
I believe the answer is:
soppy
'mawkish' is the definition.
(thesaurus)
'agent without work' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'without' means one lot of letters goes inside another ('without' can be similar in meaning to 'outside').
'work' becomes 'op' (abbreviation for opus).
'spy' placed around 'op' is 'SOPPY'.
(Other definitions for soppy that I've seen before include "Informally sentimental in sickly way" , "Wet; sentimental" , "Mawkish" , "Thoroughly wet" , "Feeble; sentimental" .)