Guide unhappy in moment of truth (8)
I believe the answer is:
showdown
'moment of truth' is the definition.
(thesaurus)
'guide unhappy' is the wordplay.
'guide' becomes 'show' (showing is a kind of guiding**).
'unhappy' becomes 'down' (I've seen this before).
'show'+'down'='SHOWDOWN'
'in' acts as a link.
(Other definitions for showdown that I've seen before include "Face-off" , "Dramatic conclusion" , "Decisive confrontation" , "Climactic finale" , "Final confrontation as in western film" .)