A speed that’s fast (5)
I believe the answer is:
apace
'fast' is the definition.
('apace' can be a synonym of 'fast')
'a speed' is the wordplay.
'speed' becomes 'pace' (pace is a kind of speed).
'a'+'pace'='APACE'
'that's' acts as a link.
(Other definitions for apace that I've seen before include "fast" , "flat out" , "Quick" , "(Move on) swiftly" , "At a rapid rate" .)