Purchase includes dry sandwich (5)
I believe the answer is:
butty
'sandwich' is the definition.
(I've seen this before)
'purchase includes dry' is the wordplay.
'purchase' becomes 'buy' ('buy' can be a synonym of 'purchase').
'includes' is an insertion indicator.
'dry' becomes 'tt' (abbreviation for teetotal).
'buy' going around 'tt' is 'BUTTY'.
(Other definitions for butty that I've seen before include "Sandwich; towed barge" , "Partner" , "something to eat" , "Sandwich; freight barge" .)