A politician visiting key American college grounds (6)
I believe the answer is:
campus
'college grounds' is the definition.
(I've seen this before)
'a politician visiting key american' is the wordplay.
'politician' becomes 'mp'.
'visiting' is an insertion indicator.
'key' becomes 'c' (I've seen this in other clues**).
'american' becomes 'us'.
'a'+'mp'='amp'
'c'+'us'='cus'
'amp' placed within 'cus' is 'CAMPUS'.
(Other definitions for campus that I've seen before include "University estate" , "university environment" , "Pre-planned college layout and grounds" , "academic haunt" , "university position" .)