Sean was mistaken in Abertawe (7)
I believe the answer is:
swansea
'abertawe' is the definition.
(I know that abertawe can be written as Swansea)
'sean was mistaken' is the wordplay.
'mistaken' indicates an anagram.
'sean'+'was'='seanwas'
'seanwas' is an anagram of 'SWANSEA'.
'in' acts as a link.
(Other definitions for swansea that I've seen before include "Welsh port" , "coastal location" , "Ecstasy, a win! (anag)" , "Birthplace of Dylan Thomas" , "South Wales city" .)