Indicator of who'll get in league after mad exploit (4,4)
I believe the answer is:
exit poll
'indicator of who'll get in' is the definition.
'league after mad exploit' is the wordplay.
'league' becomes 'l' (abbreviation).
'after' says to put letters next to each other.
'mad' indicates anagramming the letters.
'exploit' anagrammed gives 'exitpol'.
'l' after 'exitpol' is 'EXIT POLL'.
(Other definitions for exit poll that I've seen before include "Test of voters' opinion on election day" , "Survey of voters just having voted" , "Pixel lot (anag.)" , "Sample of how people voted" , "Enquiry into voters' opinions as they leave the voting place" .)