Papers upset customer wanting upper-class periodical (6)
I believe the answer is:
digest
'periodical' is the definition.
(I know that digest is a type of periodical)
'papers upset customer wanting upper-class' is the wordplay.
'papers' becomes 'ID' (identifying documents are one's 'papers').
'upset' shows that the letters should be reversed in order.
'customer wanting upper class' becomes 'gest' (I can't justify this - if you can you should believe this answer much more).
'id' back-to-front is 'di'.
'di'+'gest'='DIGEST'
Can you help me to learn more?
(Other definitions for digest that I've seen before include "Shorten" , "Think over, absorb into the mind" , "Process food" , "Abridgement" , "Absorb, in stomach or mind" .)