Subdue, in the capacity of mum (5)
I believe the answer is:
quash
'subdue' is the definition.
(I know that subdue can be written as quash)
'in the capacity of mum' is the wordplay.
'in the capacity of' becomes 'qua' (I've seen this before).
'mum' becomes 'sh' (both can be used to tell someone to be quiet).
'qua'+'sh'='QUASH'
(Other definitions for quash that I've seen before include "Invalidate" , "Subdue" , "Put down" , "Suppress -- annul" , "Suppress or crush, say a rebellion" .)