Float purchase blocked by Oscar (4)
I believe the answer is:
buoy
'float' is the definition.
(buoying is a kind of floating)
'purchase blocked by oscar' is the wordplay.
'purchase' becomes 'buy' (buy is a kind of purchase).
'blocked by' indicates putting letters inside (block can mean to plug a gap in something).
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'buy' going around 'o' is 'BUOY'.
(Other definitions for buoy that I've seen before include "Sea-lane marker" , "Floating item often marking a hazard" , "Marine float" , "keep from going under" , "To raise" .)