Most obdurate is sent to stir (8)

I believe the answer is:

stoniest

'most obdurate' is the definition.
('stony' can be a synonym of 'obdurate')

'is sent to stir' is the wordplay.
'stir' is an anagram indicator.
'is'+'sent'+'to'='issentto'
'issentto' is an anagram of 'STONIEST'.

(Other definitions for stoniest that I've seen before include "Broke more than any" , "Most petrifed" .)

I've seen this clue in The Telegraph.

Want a hint initially instead of a full solution? Install my app