Determined to follow on an attack (5)
I believe the answer is:
onset
'an attack' is the definition.
('onset' can be a synonym of 'attack')
'determined to follow on' is the wordplay.
'determined' becomes 'set' ('set' can be a synonym of 'determined').
'to follow' says to put letters next to each other.
'set' after 'on' is 'ONSET'.
(Other definitions for onset that I've seen before include "Start - attack" , "Advent" , "Start - notes (anag)" , "Storming" , "Beginning; military attack" .)