Heather collects a couple of notes for obscure topics (9)
I believe the answer is:
esoterica
'obscure topics' is the definition.
(obscure or esoteric topics)
'heather collects a couple of notes' is the wordplay.
'a couple of notes' means two substitutions for 'note'.
'heather' becomes 'erica' (the plant genus Erica includes heather plants).
'collects' is an insertion indicator.
'note' becomes 'so' (musical note).
'note' becomes 'te' ().
'so'+'te'='sote'
'erica' enclosing 'sote' is 'ESOTERICA'.
'for' is the link.
(Other definitions for esoterica that I've seen before include "Mystic objects" , "narrow fields" , "Specialized publications" , "specialist publications" , "Highly specialised subjects" .)