Pig-headed and old, at best in being treated (9)
I believe the answer is:
obstinate
'pig-headed' is the definition.
(I've seen this before)
'old at best in being treated' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'being treated' indicates anagramming the letters.
'at'+'best'+'in'='atbestin'
'atbestin' with letters rearranged gives 'bstinate'.
'o'+'bstinate'='OBSTINATE'
'and' acts as a link.
(Other definitions for obstinate that I've seen before include "Firm" , "headstrong" , "Like mules" , "Stubborn or unyielding" , "Stubborn, obdurate" .)