Foreign, but to the southwest is central Ruislip (5)
I believe the answer is:
swiss
'foreign' is the definition.
'to the southwest is central ruislip' is the wordplay.
'to the southwest' becomes 'SW' (synonyms).
'central' says to take the centre.
The middle letter of 'ruislip' is 's'.
'sw'+'is'+'s'='SWISS'
'but' is the link.
(Other definitions for swiss that I've seen before include "Tell, for example" , "Native to Geneva, say" , "Like William Tell" , "-- Cheese plant; -- roll" , "Gruyere nationality" .)