Uncle regularly follows jazz? Slander (7)
I believe the answer is:
traduce
'slander' is the definition.
(traducing is a kind of slandering)
'uncle regularly follows jazz?' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'follows' says to put letters next to each other.
'jazz?' becomes 'trad' (trad jazz is a type of jazz).
The alternating letters of 'uncle' are 'uce'.
'uce' put after 'trad' is 'TRADUCE'.
(Other definitions for traduce that I've seen before include "Calumniate" , "Asperse" , "Slander - misrepresent" , "abuse" , "Speak ill of" .)