Fast pace initially in attack (5)
I believe the answer is:
rapid
'fast' is the definition.
('rapid' can be similar in meaning to 'fast')
'pace initially in attack' is the wordplay.
'initially' suggests taking the first letters.
'in' is an insertion indicator.
'attack' becomes 'raid' (raiding is a kind of attacking).
The initial letter of 'pace' is 'p'.
'p' going within 'raid' is 'RAPID'.
(Other definitions for rapid that I've seen before include "Prompt" , "Quickly accomplished" , "Expeditious" , "Express" , "Swift, speedy" .)