Monarch embraces maturity showing enthusiasm (5)
I believe the answer is:
eager
'showing enthusiasm' is the definition.
(I've seen this in another clue)
'monarch embraces maturity' is the wordplay.
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'embraces' means one lot of letters goes inside another.
'maturity' becomes 'age' (I've seen this in another clue).
'er' going around 'age' is 'EAGER'.
(Other definitions for eager that I've seen before include "Showing keen interest" , "Like some beavers" , "Whole-hearted" , "willing" , "Longing to see" .)