American in public school or station (6)
I believe the answer is:
euston
'station' is the definition.
(London train station)
'american in public school' is the wordplay.
'american' becomes 'US' (United States).
'in' is an insertion indicator.
'public school' becomes 'eton' (Eton College).
'us' inserted within 'eton' is 'EUSTON'.
'or' is the link.
(Other definitions for euston that I've seen before include "train here?" , "rail terminal" , "No suet (anag)" , "Brown seaweed" , "North London terminus" .)