Writing up moor? Yes, primarily spacious (5)
I believe the answer is:
roomy
'spacious' is the definition.
('roomy' can be a synonym of 'spacious')
'writing up moor? yes primarily' is the wordplay.
'writing up' says the letters should be written backwards.
'primarily' suggests taking the first letters.
The initial letter of 'yes' is 'y'.
'moor' reversed gives 'room'.
'room'+'y'='ROOMY'
(Other definitions for roomy that I've seen before include "generous" , "Capacious" , "Having big space" , "Having plenty of space" , "Spacious, can swing a cat in it" .)