Vessel created with brass over long period (7)
I believe the answer is:
galleon
'vessel' is the definition.
(type of ship)
'brass over long period' is the wordplay.
'brass' becomes 'gall' (I've seen this in other clues).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'long period' becomes 'eon' (I've seen this in another clue).
'gall'+'eon'='GALLEON'
'created with' is the link.
(Other definitions for galleon that I've seen before include "Historical warship with three or more masts" , "All gone (anag.)" , "Large old sailing vessel" , "Ship of the line" , "Large Spanish sailing ship of yore" .)