Old boy, say, purchasing popular firm (9)
I believe the answer is:
obstinate
'firm' is the definition.
(I've seen this before)
'old boy say purchasing popular' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'say' becomes 'state' (synonyms).
'purchasing' indicates putting letters inside (some letters take possession of others).
'popular' becomes 'in' (something that's 'in' is fashionable).
'state' placed around 'in' is 'stinate'.
'ob'+'stinate'='OBSTINATE'
(Other definitions for obstinate that I've seen before include "Stubborn, obdurate" , "Female bird" , "Pig-headed" , "Inflexible" , "Stubborn or unyielding" .)