Stubborn old boy popular in Express (9)
I believe the answer is:
obstinate
'stubborn' is the definition.
('obstinate' can be a synonym of 'stubborn')
'old boy popular in express' is the wordplay.
'old boy' becomes 'ob'.
'popular' becomes 'in'.
'in' is an insertion indicator.
'express' becomes 'state' (stating is a kind of expressing**).
'in' inserted inside 'state' is 'stinate'.
'ob'+'stinate'='OBSTINATE'
(Other definitions for obstinate that I've seen before include "Stubborn or unyielding" , "Firm" , "Pig-headed" , "Stubbornly persistent" , "headstrong" .)