Letters Heather's about to bring in (8)
I believe the answer is:
learning
'letters' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'heather's about to bring in' is the wordplay.
'heather' becomes 'ling' (synonyms).
'about' indicates putting letters inside.
'to bring in' becomes 'earn' (as in bringing in money).
'ling' going around 'earn' is 'LEARNING'.
Can you help me to learn more?
(Other definitions for learning that I've seen before include "Large inn (anag)" , "Lore" , "Researching - scholarship" , "(Knowledge from) study" , "Acquired knowledge" .)