Something digital, mostly with quiet beat (5)
I believe the answer is:
thump
'beat' is the definition.
('thump' can be a synonym of 'beat')
'something digital mostly with quiet' is the wordplay.
'something digital' becomes 'thumb' (a digit).
'mostly' means to remove the last letter.
'with' says to put letters next to each other.
'quiet' becomes 'p' (music symbol for 'piano').
'thumb' with its last letter removed is 'thum'.
'thum'+'p'='THUMP'
(Other definitions for thump that I've seen before include "Hit (with fist)" , "Hit hard" , "Say" , "Dull heavy blow" , "Hit forcefully" .)