Brown has muzzled dog, keeping it quiet (8)
I believe the answer is:
taciturn
'quiet' is the definition.
(I've seen this in another clue)
'brown has muzzled dog keeping it' is the wordplay.
'brown' becomes 'tan' (tan is a kind of brown).
'has muzzled' means one lot of letters goes inside another.
'dog' becomes 'cur' (a cur is an aggressive or scruffy dog).
'keeping' indicates putting letters inside (some letters kept inside).
'tan' enclosing 'cur' is 'tacurn'.
'tacurn' placed around 'it' is 'TACITURN'.
(Other definitions for taciturn that I've seen before include "Tending to silence" , "Habitually uncommunicative, saying little" , "Not inclined to conversation" , "Tight-lipped" , "Quiet and uncommunicative" .)