Swallow seen in SW county mostly and old city (6)
I believe the answer is:
devour
'swallow' is the definition.
(thesaurus)
'sw county mostly and old city' is the wordplay.
'sw county' becomes 'devon' (I've seen this before).
'mostly' means to remove the last letter.
'and' is a charade indicator (letters next to each other).
'old city' becomes 'ur' (ancient city state).
'devon' with its final letter taken off is 'devo'.
'devo'+'ur'='DEVOUR'
'seen in' acts as a link.
(Other definitions for devour that I've seen before include "Eat voraciously" , "Quickly consume" , "Consume greedily" , "Swallow greedily" , "Eat (food) quickly" .)