In the capacity of quiet put down (5)
I believe the answer is:
quash
'put down' is the definition.
(both can mean to suppress or stop)
'in the capacity of quiet' is the wordplay.
'in the capacity of' becomes 'qua' (I've seen this in another clue).
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'qua'+'sh'='QUASH'
(Other definitions for quash that I've seen before include "Suppress -- annul" , "Put down" , "Overturn (judgment)" , "Invalidate" , "Subdue" .)