Catalogues keeping subset back for radicals (9)

'radicals' is the definition.
Both the definition and answer are plural nouns.
Perhaps they are linked in a way I don't understand?

'catalogues keeping subset back' is the wordplay.
'catalogues' becomes 'lists' (list can mean a catalogue).
'keeping' is an insertion indicator.
'subset' becomes 'nine' (I can't justify this - if you can you should give a lot more credence to this answer).
'back' says the letters should be written in reverse.
'nine' reversed gives 'enin'.
'lists' placed around 'enin' is 'LENINISTS'.

'for' acts as a link.

Can you help me to learn more?