Quiet expert has speed (4)
I believe the answer is:
pace
'speed' is the definition.
(pace is a kind of speed)
'quiet expert' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'expert' becomes 'ace' (ace can informally mean very good).
'p'+'ace'='PACE'
'has' acts as a link.
(Other definitions for pace that I've seen before include "Tempo, rate of moving" , "Step or tempo" , "a certain distance" , "Stride" , "Speed of walking" .)