Overwhelming defeat — challenger at last exposed (4)
I believe the answer is:
rout
'overwhelming defeat' is the definition.
(I've seen this in another clue)
'challenger at last exposed' is the wordplay.
'at last' indicates one should take the final letters.
'exposed' becomes 'out' (out on display).
The last letter of 'challenger' is 'r'.
'r'+'out'='ROUT'
(Other definitions for rout that I've seen before include "Vanquish; thrash" , "Drubbing" , "Total defeat in battle" , "Send fleeing" , "Overwhelming defeat or disorderly retreat" .)