Dead right: but not right now? (5)
I believe the answer is:
later
'not right now?' is the definition.
(I've seen this in another clue)
'dead right' is the wordplay.
'dead' becomes 'late' (similar in meaning).
'right' becomes 'r' (abbreviation).
'late'+'r'='LATER'
'but' is the link.
(Other definitions for later that I've seen before include "more modern" , "Tomorrow (or next week)?" , "In due course" , "Eventually" , "after the rest?" .)