Everyone mostly OAP in town (5)
I believe the answer is:
alloa
'town' is the definition.
(Scottish town)
'everyone mostly oap' is the wordplay.
'everyone' becomes 'all' (all people).
'mostly' means to remove the last letter.
'oap' with its final letter taken away is 'oa'.
'all'+'oa'='ALLOA'
'in' acts as a link.
(Other definitions for alloa that I've seen before include "possible cup opponent" , "Town on Firth of Forth" , "Clackmannanshire town" , "Town near Stirling" , "Scots town in the Central Lowlands" .)