Injustice fires a nun's storm (10)
I believe the answer is:
unfairness
'injustice' is the definition.
('unfairness' can be a synonym of 'injustice')
'fires a nun's storm' is the wordplay.
'storm' indicates an anagram.
'fires'+'a'+'nuns'='firesanuns'
'firesanuns' anagrammed gives 'UNFAIRNESS'.
(Other definitions for unfairness that I've seen before include "Inequity" , "quality of a brunette might you say?" , "Discrimination" .)