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' can mean two substitutions for 'creature'.
'trouble' becomes 'ail' (synonyms).
'separating' indicates putting letters inside.
'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" .)