Vessel going over a swimmer (4)
I believe the answer is:
tuna
'swimmer' is the definition.
(tuna is a kind of aquatic vertebrate)
'vessel going over a' is the wordplay.
'vessel' becomes 'tun' (tun is a kind of vessel**).
'going over' means one lot of letters go next to another.
'tun'+'a'='TUNA'
(Other definitions for tuna that I've seen before include "Warm-water food fish" , "swimmer" , "Large tasty fish, fond of a nut" , "Marine food for your strange aunt" , "Fish; edible cactus" .)