What should monarch have? Excellent beer, on reflection (7)
I believe the answer is:
regalia
'what should monarch have?' is the definition.
(regalia are ceremonial symbols of royalty)
'excellent beer on reflection' is the wordplay.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'beer' becomes 'lager' (type of beer).
'on reflection' shows that the letters should be reversed in order.
'ai'+'lager'='ailager'
'ailager' in reverse letter order is 'REGALIA'.
(Other definitions for regalia that I've seen before include "Crown jewels" , "Paraphernalia indicative of high office" , "Orb and sceptre, for example" , "Orb and sceptre etc" , "Ceremonial trimmings of high office" .)