Six sandwiches bought up for guest (7)
I believe the answer is:
visitor
'guest' is the definition.
(thesaurus)
'six sandwiches bought up' is the wordplay.
'six' becomes 'vi' ('vi' can be a synonym of 'six').
'sandwiches' becomes 'rotis'.
'bought up' says the letters should be written in reverse.
'rotis' back-to-front is 'sitor'.
'vi'+'sitor'='VISITOR'
'for' is the link.
(Other definitions for visitor that I've seen before include "Guest - tourist" , "Seasonal migratory bird" , "One coming calling" , "Caller, guest" , "She calls" .)