They produce music in more than one register (7)
I believe the answer is:
records
'they' is the definition.
Both the answer and definition are plural nouns.
Maybe they are linked in a way I don't understand?
'music in more than one register' is the wordplay.
'music in' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'more than' says to put letters next to each other.
'one register' becomes 'record' (I've seen this before).
's' put after 'record' is 'RECORDS'.
'produce' acts as a link.
Can you help me to learn more?
(Other definitions for records that I've seen before include "Archives" , "Sets down in writing or preserves in sound" , "The best (or worst) performances ever" , "Discs" , "LPs, e.g." .)