Restrain dog beginning to bark (4)
I believe the answer is:
curb
'restrain' is the definition.
(curbing is a kind of restraining)
'dog beginning to bark' is the wordplay.
'dog' becomes 'cur' (a cur is an aggressive or scruffy dog).
'beginning to' says to take the initial letters.
The first letter of 'bark' is 'b'.
'cur'+'b'='CURB'
(Other definitions for curb that I've seen before include "Constrain" , "Restrain, inhibit" , "Restrain, perhaps your enthusiasm" , "Muzzle" , "Check or restrain" .)