Races in to purchase a sandwich (5)
I believe the answer is:
butty
'a sandwich' is the definition.
(I've seen this before)
'races in to purchase' is the wordplay.
'races' becomes 'tt' (Isle of Man Tourist Trophy races).
'in' means one lot of letters goes inside another.
'to purchase' becomes 'buy' ('buy' can be a synonym of 'purchase').
'tt' placed into 'buy' is 'BUTTY'.
(Other definitions for butty that I've seen before include "Partner" , "something to eat" , "Sandwich; towed barge" , "Sandwich; freight barge" .)