Outlaw returned four-poster, say, under arrest (6)
I believe the answer is:
nabbed
'under arrest' is the definition.
(nab can mean to capture or arrest)
'outlaw returned four-poster' is the wordplay.
'outlaw' becomes 'ban' (banning is a kind of outlawing).
'returned' says the letters should be written backwards.
'four poster' becomes 'bed' (I've seen this in another clue).
'ban' written backwards gives 'nab'.
'nab'+'bed'='NABBED'
'say' is the link.
(Other definitions for nabbed that I've seen before include "Caught (in the act)" , "Caught someone doing something wrong" , "Arrested" , "Seized suddenly" .)