By quiet hesitation (3)
I believe the answer is:
per
'by' is the definition.
(I've seen this in another clue)
'quiet hesitation' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'hesitation' becomes 'er' (sound made while hesitating - um, er).
'p'+'er'='PER'
(Other definitions for per that I've seen before include "According to" , "Preposition indicating by means of" , "For each or every" , "Preposition meaning for each" .)