With due deference to a piano expert (4)
I believe the answer is:
pace
'with due deference to' is the definition.
(Latin term meaning 'with due respect to')
'a piano expert' is the wordplay.
'a piano' becomes 'p' (musical abbreviation).
'expert' becomes 'ace' (ace can informally mean very good).
'p'+'ace'='PACE'
(Other definitions for pace that I've seen before include "Tempo, rate of moving" , "Lick" , "Gait" , "With due respect" , "Stride" .)