Such gains not legit, criminal pocketing pounds (3-6)
I believe the answer is:
ill-gotten
'such gains' is the definition.
(ill-gotten gains can mean the proceeds of crime)
'not legit criminal pocketing pounds' is the wordplay.
'criminal' indicates anagramming the letters.
'pocketing' indicates putting letters inside.
'pounds' becomes 'l' (L can represent the monetary pound).
'not'+'legit'='notlegit'
'notlegit' anagrammed gives 'ilgotten'.
'ilgotten' placed around 'l' is 'ILL-GOTTEN'.
(Other definitions for ill-gotten that I've seen before include "These gains are dishonestly come by" , "Acquired by unworthy means" , "Obtained dishonestly" , "Obtained unjustly" , "Procured by evil means" .)