Mock judge rehears regularly (4)
I believe the answer is:
jeer
'mock' is the definition.
(thesaurus)
'judge rehears regularly' is the wordplay.
'judge' becomes 'j'.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'rehears' are 'eer'.
'j'+'eer'='JEER'
(Other definitions for jeer that I've seen before include "Rude, mocking remark" , "Barrack" , "Laugh at with derision" , "Catcall" , "Tease" .)