Purchase retains nothing in float (4)
I believe the answer is:
buoy
'float' is the definition.
(buoying is a kind of floating)
'purchase retains nothing' is the wordplay.
'purchase' becomes 'buy' (buy is a kind of purchase).
'retains' is an insertion indicator.
'nothing' becomes 'O' (O resembles 0).
'buy' going around 'o' is 'BUOY'.
'in' acts as a link.
(Other definitions for buoy that I've seen before include "Coloured float" , "To raise" , "Navigational marker" , "Marine float" , "Warning floater" .)