Was monarch no good to be restricted? (6)
I believe the answer is:
reined
'restricted?' is the definition.
(reining is a kind of restricting)
'was monarch no good' is the wordplay.
'was monarch' becomes 'reigned' (I've seen this before).
'no' indicates named letters should be taken away.
'good' becomes 'g' (abbreviation).
'reigned' with 'g' taken away is 'REINED'.
'to be' is the link.
(Other definitions for reined that I've seen before include "Pulled in, checked progress" , "Stopped or slowed a horse" , "Restrained" .)