Due support for the monarch (6)
I believe the answer is:
proper
'due' is the definition.
(thesaurus)
'support for the monarch' is the wordplay.
'support' becomes 'prop' (prop up can mean to support).
'for' says to put letters next to each other.
'the monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'prop'+'er'='PROPER'
(Other definitions for proper that I've seen before include "Appropriate or decorous" , "Conventional" , "Authentic" , "Correct in behaviour" , "Correct; respectable" .)