Attack on a clique or faction (5)
I believe the answer is:
onset
'attack' is the definition.
(archaic term for a military attack)
'on a clique or faction' is the wordplay.
I cannot really understand how this works, but
'on' is found in the answer.
'clique' could be 'set' (a clique is a social set) and 'set' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for onset that I've seen before include "Start - notes (anag)" , "Start - attack" , "Initial stage; attack" , "Beginning; violent attack" , "Storming" .)