Linger unhappily at stern of boat in lock (7)
I believe the answer is:
ringlet
'lock' is the definition.
(thesaurus)
'linger unhappily at stern of boat' is the wordplay.
'unhappily' indicates an anagram.
'at' is a charade indicator (letters next to each other).
'stern of' suggests the final letters (stern can mean back).
The last letter of 'boat' is 't'.
'linger' with letters rearranged gives 'ringle'.
'ringle'+'t'='RINGLET'
'in' is the link.
(Other definitions for ringlet that I've seen before include "hair styling" , "A curl - kind of tingler" , "Hair curl, of an Irish dancer perhaps" , "twist of sorts?" , "sort of lock" .)