Sally swimming to piers (7)
I believe the answer is:
riposte
'sally' is the definition.
(both can mean a witty remark)
'swimming to piers' is the wordplay.
'swimming' is an anagram indicator.
'to'+'piers'='topiers'
'topiers' anagrammed gives 'RIPOSTE'.
(Other definitions for riposte that I've seen before include "Poster I alter for repartee" , "Quick retort or return hit in fencing" , "Smart reply" , "Repartee; quick thrust (fencing)" , "Return thrust or repartee" .)