Humming sound almost entirely backing beat (6)
I believe the answer is:
larrup
'beat' is the definition.
(I know that larrup is a more specific form of the action beat)
'humming sound almost entirely backing' is the wordplay.
'humming sound' becomes 'purr' (humming sound made by a cat).
'almost' means to remove the last letter (most of the word but not all of it).
'entirely' becomes 'all' (I've seen this in another clue).
'backing' says the letters should be written in reverse.
'all' with its last letter removed is 'al'.
'purr'+'al'='purral'
'purral' written backwards gives 'LARRUP'.
(Other definitions for larrup that I've seen before include "Flog" , "Hit hard" .)