Vessel passing over a fish (4)
I believe the answer is:
tuna
'fish' is the definition.
(tuna fish is a kind of fish)
'vessel passing over a' is the wordplay.
'vessel' becomes 'tun' (tun is a kind of vessel**).
'passing over' says to put letters next to each other.
'tun'+'a'='TUNA'
(Other definitions for tuna that I've seen before include "Large salt water fish" , "Tinned food fish" , "Warm-water food fish" , "Sea creature" , "Fish sold in cans" .)