Unused vessel in Notts town (6)
I believe the answer is:
newark
'notts town' is the definition.
(I've seen this before)
'unused vessel' is the wordplay.
'unused' becomes 'new' ('new' can be similar in meaning to 'unused').
'vessel' becomes 'ark' (ark is a kind of vessel).
'new'+'ark'='NEWARK'
'in' is the link.
(Other definitions for newark that I've seen before include "US airport" , "Nottinghamshire town" , "Notts. market town" , "24 Down in Nottinghamshire" , "Notts. town where King John died" .)