A passion that’s burning (5)
I believe the answer is:
afire
'burning' is the definition.
(thesaurus)
'a passion' is the wordplay.
'passion' becomes 'fire' (fire is a kind of passion).
'a'+'fire'='AFIRE'
'that's' acts as a link.
(Other definitions for afire that I've seen before include "Alight" , "Ablaze" , "Burning" , "in flames" , "Blazing - excited" .)