Purchase includes round float (4)
I believe the answer is:
buoy
'float' is the definition.
(buoying is a kind of floating)
'purchase includes round' is the wordplay.
'purchase' becomes 'buy' ('buy' can be a synonym of 'purchase').
'includes' indicates putting letters inside.
'round' becomes 'o' (round can mean a round object**).
'buy' placed around 'o' is 'BUOY'.
(Other definitions for buoy that I've seen before include "Coloured float" , "keep from going under" , "Warning float" , "dolphin" , "Anchored (navigation) float" .)