Quick but poor start in an attack (5)
I believe the answer is:
rapid
'quick' is the definition.
(synonyms)
'poor start in an attack' is the wordplay.
'start' says to take the initial letters.
'in' means one lot of letters goes inside another.
'an attack' becomes 'raid' (raid is a kind of attack).
The initial letter of 'poor' is 'p'.
'p' put into 'raid' is 'RAPID'.
'but' acts as a link.
(Other definitions for rapid that I've seen before include "moving fast" , "Expeditious" , "Flying" , "Zippy" , "Precipitate" .)