Doctor Who's day to have confrontation (8)
I believe the answer is:
'confrontation' is the definition.
('showdown' can be a synonym of 'confrontation')
'doctor who's day to have' is the wordplay.
'doctor' indicates anagramming the letters.
'day' becomes 'd'.
'to have' becomes 'own' ('own' can be a synonym of 'have').
'whos' anagrammed gives 'show'.
(Other definitions for showdown that I've seen before include "Dramatic conclusion" , "Face-off" , "Denouement" , "Climactic finale" , "Final test, often found in westerns" .)