To support monarch is correct (6)
I believe the answer is:
proper
'is correct' is the definition.
(associated in meaning)
'to support monarch' is the wordplay.
'to support' becomes 'prop' (prop is a kind of support).
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'prop'+'er'='PROPER'
(Other definitions for proper that I've seen before include "Conforming to etiquette" , "Suitable and correct" , "Appropriate or decorous" , "well-behaved" , "Refined" .)