Be determined to attack (5)
I believe the answer is:
beset
'attack' is the definition.
(besetting is a kind of attacking)
'be determined' is the wordplay.
'determined' becomes 'set' (similar in meaning).
'be'+'set'='BESET'
'to' is the link.
(Other definitions for beset that I've seen before include "Assail or attack on all sides" , "Harry" , "Hemmed in or surrounded" , "besieged" , "Plagued" .)