Quick but soft-hearted attack! (5)
I believe the answer is:
rapid
'quick' is the definition.
('rapid' can be a synonym of 'quick')
'soft-hearted attack' is the wordplay.
I cannot quite understand how this works, but
'soft' could be 'p' (musical abbreviation for 'piano') and 'p' is located in the answer.
'attack' could be 'raid' (raiding is a kind of attacking) and 'raid' is located in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
'but' is the link.
Can you help me to learn more?
(Other definitions for rapid that I've seen before include "Zippy" , "Electrical connector" , "Showing pace" , "Speedy, quick" , "Quickly accomplished" .)