Vessel opening after day (7)
I believe the answer is:
frigate
'vessel' is the definition.
(frigate is a kind of vessel)
'opening after day' is the wordplay.
'opening' becomes 'gate' (a gate is an opening or entrance).
'after' is a charade indicator (letters next to each other).
'day' becomes 'fri' (abbreviation for Friday).
'gate' after 'fri' is 'FRIGATE'.
(Other definitions for frigate that I've seen before include "Light warship" , "Ship" , "Naval escort vessel" , "Warship - seabird" , "Warship; -- bird" .)