Doctor who's on deck for decisive engagement (8)
I believe the answer is:
showdown
'decisive engagement' is the definition.
'doctor who's on deck' is the wordplay.
'doctor' indicates an anagram.
'on' means one lot of letters go next to another.
'deck' becomes 'down' (**).
'whos' is an anagram of 'show'.
'show'+'down'='SHOWDOWN'
'for' acts as a link.
(Other definitions for showdown that I've seen before include "Final confrontation as in western film" , "Dramatic conclusion" , "Decisive confrontation" , "Face-off" , "Climactic finale" .)