Pig-headed old boys can eat out (9)
I believe the answer is:
obstinate
'pig-headed' is the definition.
(I've seen this before)
'old boys can eat out' is the wordplay.
'old boys' becomes 'obs' (I've seen this in other clues).
'can' becomes 'tin' ('tin' can be a synonym of 'can').
'out' is an anagram indicator (out can mean wrong or inaccurate).
'eat' with letters rearranged gives 'ate'.
'obs'+'tin'+'ate'='OBSTINATE'
(Other definitions for obstinate that I've seen before include "Stubborn, obdurate" , "Mulish" , "Like mules" , "Female bird" , "Cussed" .)