Swift attack captures leader of pirates (5)
I believe the answer is:
rapid
'swift' is the definition.
(thesaurus)
'attack captures leader of pirates' is the wordplay.
'attack' becomes 'raid' (raiding is a kind of attacking).
'captures' means one lot of letters goes inside another.
'leader of' suggests taking the first letters.
The first letter of 'pirates' is 'p'.
'raid' placed around 'p' is 'RAPID'.
(Other definitions for rapid that I've seen before include "Zippy" , "Express" , "Precipitate" , "Fast, speedy" , "Speedy (5)" .)