Exploit unusually large survey of voters (4,4)
I believe the answer is:
exit poll
'survey of voters' is the definition.
(an exit poll asks how people have voted)
'exploit unusually large' is the wordplay.
'unusually' indicates an anagram.
'large' becomes 'L' (eg in clothes sizes).
'exploit' with letters rearranged gives 'exitpol'.
'exitpol'+'l'='EXIT POLL'
(Other definitions for exit poll that I've seen before include "Sample of how people voted" , "Pixel lot (anag.)" , "Sampling of voters after voting" , "Test of voters' opinion on election day" , "Survey of voters just having voted" .)