Top speed briefly rising (3)
I believe the answer is:
cap
'top' is the definition.
(both can mean to beat or outdo)
'speed briefly rising' is the wordplay.
'speed' becomes 'pace' (pace is a kind of speed).
'briefly' means to remove the last letter.
'rising' shows that the letters should be reversed in order (letters written upwards in a down clue).
'pace' with its last letter removed is 'pac'.
'pac' in reverse letter order is 'CAP'.
(Other definitions for cap that I've seen before include "Go with this in hand to be obsequious" , "One on head; excel" , "Beat; upper limit" , "subsidy regime" , "Headgear item" .)