Vessel from Oslo, possibly, at the pierhead (5)
I believe the answer is:
sloop
'vessel' is the definition.
(thesaurus)
'oslo possibly at the pierhead' is the wordplay.
'possibly' is an anagram indicator.
'at' says to put letters next to each other.
'the pierhead' becomes 'p' (head letter of 'pier').
'oslo' is an anagram of 'sloo'.
'sloo'+'p'='SLOOP'
'from' acts as a link.
(Other definitions for sloop that I've seen before include "Single-masted sailing vessel" , "Vessel with a single mast" , "One-masted sailing boat" , "Type of sailing boat" , "One-masted sailing ship" .)