Pure space around capital (9)
I believe the answer is:
upper case
'capital' is the definition.
('upper case' can be a synonym of 'capital')
'pure space around' is the subsidiary indication.
'around' indicates anagramming the letters.
'pure'+'space'='purespace'
'purespace' with letters rearranged gives 'uppercase'.
(Other definitions for upper case that I've seen before include "LSD, but not pot, is in this" , "(capitals here, please)" , "characters starting sentences?" , "used to begin sentence" , "Formal description of capital letters" .)