Trouble between animal and insect, one will attack (9)
I believe the answer is:
assailant
'one will attack' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'trouble between animal and insect' is the wordplay.
'trouble' becomes 'ail' (synonyms).
'between' indicates putting letters inside.
'animal' becomes 'ass' (I've seen this before).
'and' is a charade indicator (letters next to each other).
'insect' becomes 'ant' (ant is a kind of insect).
'ass'+'ant'='assant'
'ail' put into 'assant' is 'ASSAILANT'.
Can you help me to learn more?
(Other definitions for assailant that I've seen before include "Who would tear into" , "Attacking person" , "who is on the offensive?" , "Aggressive sort" , "One attacking another" .)