Unpleasant gang of criminals briefly returned to infiltrate racket (7)
I believe the answer is:
noisome
'unpleasant' is the definition.
(I've seen this before)
'briefly returned to infiltrate racket' is the wordplay.
'briefly' becomes 'mo' (I can't explain this - if you can you should believe this answer much more).
'returned' says the letters should be written in reverse.
'to infiltrate' indicates putting letters inside.
'racket' becomes 'noise' (racket can mean a loud noise).
'mo' in reverse letter order is 'om'.
'om' placed within 'noise' is 'NOISOME'.
'gang of criminals' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for noisome that I've seen before include "Repellent" , "Disgusting -- ie moons (anag)" , "Offensively smelly" , "Smelling badly" , "Foul; putrid" .)