Dog brush (4)
I believe the answer is:
tail
I believe this clue is a double definition.
'dog' is the first definition.
('tail' can be a synonym of 'dog')
'brush' is the second definition.
(thesaurus)
(Other definitions for tail that I've seen before include "Caudal fin" , "private eye" , "Comet's luminous trail" , "Follow (a suspect)" , "that can be cast by a lizard" .)