Mock gross judge (4-7)
I believe the answer is:
make-believe
'mock' is the definition.
(thesaurus)
'gross judge' is the wordplay.
'gross' becomes 'make' (as in grossing or making a certain amount of money).
'judge' becomes 'believe' (believing is a kind of judging).
'make'+'believe'='MAKE-BELIEVE'
(Other definitions for make-believe that I've seen before include "Fantasy" , "Pretence" .)