Agree with beginning to restrain a monster (7)
I believe the answer is:
chimera
'monster' is the definition.
(Chimera is an example)
'agree with beginning to restrain a' is the wordplay.
'agree' becomes 'chime'.
'with' is a charade indicator (letters next to each other).
'beginning to' says to take the initial letters.
The first letter of 'restrain' is 'r'.
'chime'+'r'+'a'='CHIMERA'
(Other definitions for chimera that I've seen before include "phantom" , "wild dream" , "Fantastic creation of the mind, like Greek mythical monster" , "Lion-headed monster" , "eaders of radio appear after jingle" .)