There's a joke in here - it's a good one! (7)
I believe the answer is:
cracker
'a good one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'there's a joke in here' is the wordplay.
'there's a joke' becomes 'crack' (a crack is a joke or quip).
'in here' becomes 'er' (I am not sure about this - if you are sure you should believe this answer much more).
'crack'+'er'='CRACKER'
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for cracker that I've seen before include "Biscuit - banger - brilliant one" , "Knockout biscuit?" , "it may be specially for Christmas" , "Bonbon" , "pulled at Christmas?" .)