Control power of monarch in speech (4)
I believe the answer is:
rein
'control' is the definition.
(I know that rein is a type of control)
'power of monarch in speech' is the wordplay.
'power of monarch' becomes 'reign' (a monarch reigns).
'in speech' shows a homophone (sound like).
'reign' sounds like 'REIN'.
(Other definitions for rein that I've seen before include "Long strap to restrain" , "Restraint; strap attached to horse's bit" , "given free to experienced riders?" , "Restraining lead" , "Horse controller" .)