Let aunt off, accepting kiss that's joyous (8)
I believe the answer is:
exultant
'joyous' is the definition.
(thesaurus)
'let aunt off accepting kiss' is the wordplay.
'off' indicates an anagram.
'accepting' indicates putting letters inside (inserted letters are accepted).
'kiss' becomes 'x' (X can represent a kiss).
'let'+'aunt'='letaunt'
'letaunt' is an anagram of 'eultant'.
'eultant' placed around 'x' is 'EXULTANT'.
'that's' is the link.
(Other definitions for exultant that I've seen before include "Jubilant, rejoicing greatly" , "really happy" , "Triumphantly elated" , "Intensely joyful" , "Triumphant" .)