Drive back and swim round the pierhead (5)
I believe the answer is:
repel
'drive back' is the definition.
('repel' can be a synonym of 'drive back')
'swim round the pierhead' is the wordplay.
'swim' becomes 'reel' (I've seen this before).
'round' is an insertion indicator.
'the pierhead' becomes 'p' (head letter of 'pier').
'reel' placed around 'p' is 'REPEL'.
'and' acts as a link.
(Other definitions for repel that I've seen before include "Be impervious to" , "Drive back or ward off" , "Fight off - drive back" , "Fend off" , "Fill with distaste" .)