Warm up again before missing first qualifying round (6)
I believe the answer is:
reheat
'warm up again' is the definition.
(to heat or warm up again)
'before missing first qualifying round' is the wordplay.
'before' becomes 'pre' (prefix meaning 'before').
'missing first' means to remove the first letter.
'qualifying round' becomes 'heat' (I've seen this in another clue).
'pre' with its first letter taken away is 're'.
're'+'heat'='REHEAT'
(Other definitions for reheat that I've seen before include "Cook again" , "Method of getting extra power from jet engine" , "Process increasing jet engine thrust" , "Of cooked food, give another go in the microwave" , "Warm (food) up again" .)