Float to purchase a container for oxygen (4)
I believe the answer is:
buoy
'float' is the definition.
(I know that buoy is a more specific form of the action float)
'purchase a container for oxygen' is the wordplay.
'purchase' becomes 'buy' (buy is a kind of purchase).
'a container for' means one lot of letters goes inside another.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'buy' placed around 'o' is 'BUOY'.
'to' acts as a link.
(Other definitions for buoy that I've seen before include "Floating sea marker" , "Floating navigation mark" , "Marine float" , "Sea-lane marker" , "Warning float" .)