Ultimately less cold, becoming vivacious (6)
I believe the answer is:
sparky
'becoming vivacious' is the definition.
I can't judge whether this defines the answer.
'ultimately less cold' is the wordplay.
'ultimately less' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'cold' becomes 'parky' (similar in meaning).
's'+'parky'='SPARKY'
Can you help me to learn more?
(Other definitions for sparky that I've seen before include "Lively, high-spirited" , "Clever and enjoyable to be with" , "Spirited" , "jaunty" .)