Great speed but no power (3)
I believe the answer is:
ace
'great' is the definition.
(I've seen this before)
'speed but no power' is the wordplay.
'speed' becomes 'pace' (pace is a kind of speed).
'but no' suggests deleting specific letters.
'power' becomes 'p' (symbol in physics).
'pace' with 'p' removed is 'ACE'.
(Other definitions for ace that I've seen before include "One in playing-cards" , "single > club > champion" , "Card with a single pip" , "One in a suit" , "Playing card with single spot" .)