Size of crowd entering at Everton needs greatly reducing (4)
I believe the answer is:
gate
'size of crowd' is the definition.
(gate can mean spectators at a sporting event)
'entering at everton needs greatly reducing' is the wordplay.
'needs greatly reducing' indicates a hidden word.
'GATE' is hidden within 'entering at everton'.
(Other definitions for gate that I've seen before include "Attendance figure" , "Airport boarding point" , "Narrow opening" , "Attendance; entrance" , "Garden door?" .)