Oxygen exercises used on speed run (7)
I believe the answer is:
operate
'run' is the definition.
('operate' can be a synonym of 'run')
'oxygen exercises used on speed' is the wordplay.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'exercises' becomes 'pe'.
'used on' is a charade indicator (letters next to each other).
'speed' becomes 'rate' (rate is a kind of speed**).
'o'+'pe'+'rate'='OPERATE'
(Other definitions for operate that I've seen before include "Make function" , "Carry out surgery" , "Act, like a surgeon" , "Control, run, a mcahine say" , "Function; run" .)