Old boy looked closely but conformed (6)
I believe the answer is:
obeyed
'conformed' is the definition.
(obeying is a kind of conforming)
'old boy looked closely' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'looked closely' becomes 'eyed' (eyeing is a kind of looking. I am not sure about the 'closely' bit.).
'ob'+'eyed'='OBEYED'
'but' is the link.
Can you help me to learn more?
(Other definitions for obeyed that I've seen before include "Carried out someone's instructions" , "Complied with demands" , "See 8" , "Conformed" , "Responded to an order" .)