Attacker in trouble separating two creatures (9)
I believe the answer is:
assailant
'attacker' is the definition.
(synonyms)
'trouble separating two creatures' is the wordplay.
'two creatures' means two substitutions for 'creature'.
'trouble' becomes 'ail' (synonyms).
'separating' is an insertion indicator.
'creature' becomes 'ass' (donkey).
'creature' becomes 'ant'.
'ass'+'ant'='assant'
'ail' put into 'assant' is 'ASSAILANT'.
'in' acts as a link.
(Other definitions for assailant that I've seen before include "Aggressive sort" , "Attacking person" , "Who would tear into" , "Aggressor" , "One attacking another" .)