Second in reality showdown (6,2,5)
I believe the answer is:
moment of truth
'showdown' is the definition.
(term for a showdown or confrontation)
'second in reality' is the wordplay.
'second' becomes 'moment' (synonyms).
'in reality' becomes 'of truth' (truthful or real).
'moment'+'oftruth'='MOMENT OF TRUTH'
(Other definitions for moment of truth that I've seen before include "spin-doctor aims to avoid?" , "From the mutton (anag) - significant point in time" , "Crucial point in time on which much depends" , "Crisis time" , "Crunch" .)