Agree to be enthusiastic (5)
I believe the answer is:
eager
'enthusiastic' is the definition.
(similar in meaning)
'agree to be' is the wordplay.
'to be' indicates anagramming the letters.
'agree' with letters rearranged gives 'EAGER'.
(Other definitions for eager that I've seen before include "Earnest" , "Enthusiastic, raring to go" , "Impatient" , "Longing to see" , "Excited by desire" .)