Turn into beast, being stubborn (9)
I believe the answer is:
obstinate
'stubborn' is the definition.
('obstinate' can be a synonym of 'stubborn')
'turn into beast' is the wordplay.
'turn' indicates an anagram.
'into'+'beast'='intobeast'
'intobeast' anagrammed gives 'OBSTINATE'.
'being' acts as a link.
(Other definitions for obstinate that I've seen before include "Pig-headed" , "Firm" , "Mulish" , "such as a mule" , "Stubbornly persistent" .)