One enthralled by a simple finale of song in concert (8)
I believe the answer is:
agreeing
'in concert' is the definition.
(thesaurus)
'one enthralled by a simple finale of song' is the wordplay.
'one' becomes 'i' (Roman numeral).
'enthralled by' means one lot of letters goes inside another ('enthral' can mean 'capture').
'simple' becomes 'green' (I can't justify this - if you can you should give a lot more credence to this answer).
'finale of' suggests the final letters.
The last letter of 'song' is 'g'.
'a'+'green'='agreen'
'i' put into 'agreen' is 'agreein'.
'agreein'+'g'='AGREEING'
Can you help me to learn more?
(Other definitions for agreeing that I've seen before include "Holding same opinion" , "In accordance with" , "In concurrence" , "Concurring" , "OK" .)