Roger sags terribly, seeing attacker (9)
I believe the answer is:
aggressor
'attacker' is the definition.
(synonyms)
'roger sags terribly' is the wordplay.
'terribly' is an anagram indicator.
'roger'+'sags'='rogersags'
'rogersags' anagrammed gives 'AGGRESSOR'.
'seeing' is the link.
(Other definitions for aggressor that I've seen before include "The one who starts the fight" , "One attacking without provocation" , "Assailant" , "Hostile attacker" , "One acting hostilely" .)