Jests go off as they grow old (8)
I believe the answer is:
badinage
'jests go' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'off as they grow old' is the wordplay.
I cannot really understand how this works, but
'off' could be 'bad' (I've seen this in other clues) and 'bad' is found within the answer.
'as' could be 'in' (I've seen this in other clues) and 'in' is located in the answer.
'grow old' could be 'age' (synonyms) and 'age' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for badinage that I've seen before include "Humorous banter" , "Playful banter" , "Ribbing" , "Wordplay" , "Raillery, give-and-take" .)