Finally telling a good joke (3)
I believe the answer is:
gag
'joke' is the definition.
(gagging is a kind of joking)
'finally telling a good' is the wordplay.
'finally' suggests the final letters.
'a good' becomes 'ag' (I've seen this in other clues).
The last letter of 'telling' is 'g'.
'g'+'ag'='GAG'
(Other definitions for gag that I've seen before include "Stifle; joke" , "Put a muzzle on the joke" , "that renders one speechless" , "Funny" , "Prevent free expression" .)