Enthusiastic period enshrined by former monarch (5)
I believe the answer is:
eager
'enthusiastic' is the definition.
(similar in meaning)
'period enshrined by former monarch' is the wordplay.
'period' becomes 'age' (age is a kind of period).
'enshrined by' is an insertion indicator (some letters are held or enshrined by others).
'former monarch' becomes 'ER' (Elizabeth Regina, ie Queen Elizabeth).
'age' placed within 'er' is 'EAGER'.
(Other definitions for eager that I've seen before include "Ardent" , "Full of enthusiasm" , "passionate" , "Agree (anag.)" , "Desperate" .)