Move quickly when the heats are rearranged before the end of the competition (6)
I believe the answer is:
hasten
'move quickly' is the definition.
(I've seen this before)
'the heats are rearranged before the end of the competition' is the wordplay.
'are rearranged' indicates anagramming the letters.
'before' is a charade indicator (letters next to each other).
'the end of the' indicates one should take the final letters.
The last letter of 'competition' is 'n'.
'heats' anagrammed gives 'haste'.
'haste'+'n'='HASTEN'
'when' acts as a link.
(Other definitions for hasten that I've seen before include "Rush" , "Travel with expedition" , "race? - with 26a" , "Step up" , "Hurry along" .)