Stubborn old boy's dined, purchasing can (9)
I believe the answer is:
obstinate
'stubborn' is the definition.
(I know that stubborn can be written as obstinate)
'old boy's dined purchasing can' is the wordplay.
'old boy's' becomes 'OBs' (abbreviation for a private school 'old boy').
'dined' becomes 'ate' (I've seen this in another clue).
'purchasing' indicates putting letters inside (some letters take possession of others).
'can' becomes 'tin' (both are metal containers).
'obs'+'ate'='obsate'
'obsate' enclosing 'tin' is 'OBSTINATE'.
(Other definitions for obstinate that I've seen before include "Inflexible" , "Like mules" , "Stubborn or unyielding" , "Female bird" , "such as a mule" .)