Foolish joke by Mary Jane (8)
I believe the answer is:
crackpot
'foolish' is the definition.
(I've seen this before)
'joke by mary jane' is the wordplay.
'joke' becomes 'crack' (a crack is a joke or quip).
'by' means one lot of letters go next to another.
'mary jane' becomes 'pot' (synonyms).
'crack'+'pot'='CRACKPOT'
(Other definitions for crackpot that I've seen before include "Eccentric; impractical" , "Mad" , "Foolish" , "Crazy or impractical person" , "Fruitcake, screwball" .)