Discernment one Saint displayed in hours of darkness (7)
I believe the answer is:
insight
'discernment' is the definition.
(insight is a kind of discernment)
'one saint displayed in hours of darkness' is the wordplay.
'one' becomes 'i' (Roman numeral).
'saint' becomes 's'.
'displayed in' is an insertion indicator.
'hours of darkness' becomes 'night' (I've seen this before).
's' placed within 'night' is 'nsight'.
'i'+'nsight'='INSIGHT'
(Other definitions for insight that I've seen before include "Penetrative quality" , "Practical knowledge" , "Intuitive understanding" , "Imaginative penetration" , "Grasp" .)