Mother-in-law perhaps initially disappearing -- joy! (7)
I believe the answer is:
elation
'joy' is the definition.
(elation is a kind of joy)
'mother-in-law perhaps initially disappearing' is the wordplay.
'mother-in-law perhaps' becomes 'relation' (mother-in-law is a kind of relation).
'initially disappearing' means to remove the first letter.
'relation' with its initial letter removed is 'ELATION'.
(Other definitions for elation that I've seen before include "Bliss" , "Exaltation" , "Feeling of delight or pride" , "Feeling of intense delight" , "Euphoria of toenail" .)