Quick progress initially in military assault (5)
I believe the answer is:
rapid
'quick' is the definition.
(synonyms)
'progress initially in military assault' is the wordplay.
'initially' says to take the initial letters.
'in' is an insertion indicator.
'military assault' becomes 'raid' (I've seen this in another clue).
The first letter of 'progress' is 'p'.
'p' placed into 'raid' is 'RAPID'.
(Other definitions for rapid that I've seen before include "Prompt" , "Moving at speed" , "Happening at great speed" , "Expeditious" , "Flying" .)