Snake gets head of tuna in vessel (4)
I believe the answer is:
boat
'vessel' is the definition.
(boat is a kind of vessel)
'snake gets head of tuna' is the wordplay.
'snake' becomes 'boa' (boa is a kind of snake).
'gets' says to put letters next to each other.
'head of' indicates taking the first letters.
The initial letter of 'tuna' is 't'.
'boa'+'t'='BOAT'
'in' acts as a link.
(Other definitions for boat that I've seen before include "See 18" , "Water craft" , "Floating vessel" , "Small ship" , "Water (or gravy) vessel" .)