Such gains not legit, criminally trousering fifty? (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 criminally trousering fifty?' is the wordplay.
'criminally' indicates an anagram (I've seen this in other clues).
'trousering' indicates putting letters inside (trouser can mean to steal).
'fifty?' becomes 'L' (Roman numeral).
'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 obtained by unlawful means" , "These gains are through evil or unlawful means" , "Acquired by unworthy means" , "These gains are the product of crime - need telling to" , "Obtained wrongly or criminally - give telling to" .)