Leaders of Newcastle usually taunt Sunderland fans (4)
I believe the answer is:
nuts
'fans' is the definition.
(both can mean enthusiasts)
'leaders of newcastle usually taunt sunderland' is the wordplay.
'leaders of' indicates taking the first letters.
The first letters of 'newcastle usually taunt sunderland' is 'NUTS'.
(Other definitions for nuts that I've seen before include "Crazy (sl.)" , "Beans" , "out to lunch" , "Edible seeds with hard shells" , "off one's trolley" .)