It's nearly all about Dartmoor (5)
I believe the answer is:
devon
'dartmoor' is the definition.
Both the answer and definition are singular nouns.
Maybe there's an association between them I don't understand?
'it's nearly all about' is the wordplay.
'it's nearly all' becomes 'dev' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' becomes 'on' (as in a book 'on' a certain topic).
'dev'+'on'='DEVON'
Can you help me to learn more?
(Other definitions for devon that I've seen before include "County in SW England" , "Raleigh's county" , "Clotted-cream county" , "West country county" , "English county" .)