Some light-hearted works, or a septet should be arranged (9)
I believe the answer is:
operettas
'some light-hearted works' is the definition.
Although both the answer and definition are plural nouns, I can't see how one could define the other.
'or a septet should be arranged' is the wordplay.
'should be arranged' indicates an anagram.
'or'+'a'+'septet'='oraseptet'
'oraseptet' with letters rearranged gives 'OPERETTAS'.
Can you help me to learn more?
(Other definitions for operettas that I've seen before include "Performances" , "Light musicals for poetaster" , "Musical compositions as by Gilbert & Sullivan" , "10d [IOLANTHE], 45d [RUDDIGORE] and 6a [PRINCESS IDA], say" .)