Quick raid capturing the pierhead (5)
I believe the answer is:
rapid
'quick' is the definition.
(synonyms)
'raid capturing the pierhead' is the wordplay.
'capturing' means one lot of letters goes inside another.
'the pierhead' becomes 'p' (head letter of 'pier').
'raid' enclosing 'p' is 'RAPID'.
(Other definitions for rapid that I've seen before include "Fast, quick" , "Swift, speedy" , "Showing pace" , "Precipitate" , "Happening quickly" .)