Vessel returning in long boat (7)
I believe the answer is:
pinnace
'boat' is the definition.
(historical type of small boat)
'vessel returning in long' is the wordplay.
'vessel' becomes 'can' (I've seen this before).
'returning' is a reversal indicator.
'in' indicates putting letters inside.
'long' becomes 'pine' (synonyms).
'can' in reverse letter order is 'nac'.
'nac' placed inside 'pine' is 'PINNACE'.
(Other definitions for pinnace that I've seen before include "Vessel" , "Ship's boat - nice nap (anag)" , "tender" , "Small boat" .)