Joy about even 50% rise (9)
I believe the answer is:
elevation
'rise' is the definition.
(elevation is a kind of rise)
'joy about even 50' is the wordplay.
'joy' becomes 'elation' (elation is a kind of joy**).
'about' is an insertion indicator.
'i0' suggests halving the letters.
'even' cut in half is 'ev'.
'elation' going around 'ev' is 'ELEVATION'.
(Other definitions for elevation that I've seen before include "scale drawing" , "Height above sea level" , "Leave it on for altitude of a place" , "Architectural drawing" , "High point" .)