With complaint, over the top, dipped into information obtained illegally (3-6)
I believe the answer is:
ill-gotten
'obtained illegally' is the definition.
(as in ill-gotten gains)
'with complaint over the top dipped into information' is the wordplay.
'with' means one lot of letters go next to another.
'complaint' becomes 'ill' (synonyms).
'over the top' becomes 'OTT' (abbreviation).
'dipped into' indicates putting letters inside.
'information' becomes 'gen' (gen can informally mean information).
'ott' going within 'gen' is 'gotten'.
'ill' next to 'gotten' is 'ILL-GOTTEN'.
(Other definitions for ill-gotten that I've seen before include "These gains are dishonestly come by" , "Obtained dishonestly" , "These gains are the product of crime - need telling to" , "Acquired by unworthy means" , "Procured by evil means" .)