Permitted to be inspired by whimsical joy (8)
I believe the answer is:
felicity
'joy' is the definition.
(thesaurus)
'permitted to be inspired by whimsical' is the wordplay.
'permitted' becomes 'licit' (I've seen this before).
'to be inspired by' means one lot of letters goes inside another.
'whimsical' becomes 'fey' (I've seen this before).
'licit' put within 'fey' is 'FELICITY'.
(Other definitions for felicity that I've seen before include "Joy -- eloquence" , "Girl" , "Intense joy" , "Complete happiness" , "happy event" .)