Judge relative to be sprightly (6)
I believe the answer is:
jaunty
'sprightly' is the definition.
(thesaurus)
'judge relative' is the wordplay.
'judge' becomes 'j'.
'relative' becomes 'aunty' (aunty is a kind of relative).
'j'+'aunty'='JAUNTY'
'to be' is the link.
(Other definitions for jaunty that I've seen before include "Cheerfully self-confident" , "Sprightly and cheerful" , "Happy" , "Cheerful and self-confident, perhaps with a swagger" , "Stylish" .)