Vehicle to buy, horse included (5)
I believe the answer is:
buggy
'vehicle' is the definition.
(buggy is a kind of vehicle)
'buy horse included' is the wordplay.
'horse' becomes 'gg'.
'included' is an insertion indicator.
'buy' placed around 'gg' is 'BUGGY'.
'to' is the link.
(Other definitions for buggy that I've seen before include "Of software, containing errors" , "Small open motor vehicle; infested" , "A small vehicle as for baby" , "Open-topped vehicle" , "Light horse-drawn vehicle; infested" .)