Once out, turn, furious but supportive (11)
I believe the answer is:
encouraging
'supportive' is the definition.
(similar in meaning)
'once out turn furious' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'turn' becomes 'u' (a 'U-turn' is a kind of turn).
'furious' becomes 'raging' (synonyms).
'once' with letters rearranged gives 'enco'.
'enco'+'u'+'raging'='ENCOURAGING'
'but' is the link.
(Other definitions for encouraging that I've seen before include "Promising, optimistic" , "Giving hope or confidence to" .)