Former pupil wearing clothes I'm obliged to get (6)
I believe the answer is:
obtain
'get' is the definition.
(obtaining is a kind of getting)
'former pupil wearing clothes i'm' is the wordplay.
'former pupil' becomes 'ob' (abbreviation for 'Old Boy').
'wearing' becomes 'in' (wear in (and 'in' is fashionable)).
'clothes' indicates putting letters inside.
'i'm' becomes 'ta' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ob'+'in'='obin'
'obin' enclosing 'ta' is 'OBTAIN'.
'obliged to' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for obtain that I've seen before include "Get one's hands on" , "Get - secure" , "Gain possession of" , "Acquire, secure" , "Get, procure" .)