While accompanying special forces, offend a killer (8)
I believe the answer is:
assassin
'a killer' is the definition.
(assassin is a kind of killer)
'while accompanying special forces offend' is the wordplay.
'while' becomes 'as' (eg 'while I walked' means 'as I walked').
'accompanying' says to put letters next to each other.
'special forces' becomes 'SAS' (Special Air Service).
'offend' becomes 'sin' (sin can mean to commit an offence).
'as'+'sas'+'sin'='ASSASSIN'
(Other definitions for assassin that I've seen before include "butcher" , "Carlos the Jackal was one" , "Terrorist" , "Hitman" , "One making hits" .)