Hold back aggressive dog beginning to bark (4)
I believe the answer is:
curb
'hold back' is the definition.
(thesaurus)
'aggressive dog beginning to bark' is the wordplay.
'aggressive dog' becomes 'cur' (I've seen this before).
'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 "Strap used to restrain horse" , "Restrain, perhaps your enthusiasm" , "Bridle" , "[LIMITATION]" , "Hold back or try to prevent" .)