Gives swimmer greeting (9)
I believe the answer is:
handshake
'greeting' is the definition.
(thesaurus)
'gives swimmer' is the wordplay.
'gives' becomes 'hands' ('hand' can be a synonym of 'give').
'swimmer' becomes 'hake' (hake is a kind of aquatic vertebrate**).
'hands'+'hake'='HANDSHAKE'
(Other definitions for handshake that I've seen before include "Friendly gesture" , "acknowledgment" , "Gesture of greeting or of agreeing a deal" , "A link" , "Physical greeting" .)