Most convivial male monarch, one in repose (8)
I believe the answer is:
merriest
'most convivial' is the definition.
(convivial can mean merry or cheerful)
'male monarch one in repose' is the wordplay.
'male' becomes 'm' (common abbreviation).
'monarch' becomes 'er' (abbreviation for Elizabeth Regina).
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'repose' becomes 'rest' (synonyms).
'i' inserted within 'rest' is 'riest'.
'm'+'er'+'riest'='MERRIEST'
(Other definitions for merriest that I've seen before include "higher than the rest" , "most tipsy" , "Most intoxicated?" , "Jolliest" , "Most jolly and high spirited" .)