Stuff oneself before boarding old vessel (7)
I believe the answer is:
overeat
'stuff oneself' is the definition.
(I've seen this before)
'before boarding old vessel' is the wordplay.
'before' becomes 'ere' (usually literary).
'boarding' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'vessel' becomes 'vat' (vat is a kind of vessel**).
'o'+'vat'='ovat'
'ere' placed within 'ovat' is 'OVEREAT'.
(Other definitions for overeat that I've seen before include "Indulge in gluttony" , "pig out?" , "Take in excess" , "Take too much food" , "Consume too much food, stuff your stomach" .)