Appropriate to support Her Majesty (6)
I believe the answer is:
proper
'appropriate' is the definition.
(similar in meaning)
'support her majesty' is the wordplay.
'support' becomes 'prop' (prop up can mean to support).
'her majesty' becomes 'ER' (Elizabeth Regina).
'prop'+'er'='PROPER'
'to' acts as a link.
(Other definitions for proper that I've seen before include "Real; genuine" , "Conventional" , "Correct; respectable" , "Actual" , "Refined" .)