Upset American uncle refers to deity and various religious doctrines (6)
I believe the answer is:
dogmas
'religious doctrines' is the definition.
(dogma is a kind of religious doctrine)
'upset american uncle refers to' is the wordplay.
'upset' says the letters should be written in reverse.
'american uncle' becomes 'Sam' (Uncle Sam is a nickname for the USA).
'refers to' becomes 'god' (I can't justify this - if you can you should believe this answer much more).
'sam'+'god'='samgod'
'samgod' written backwards gives 'DOGMAS'.
'deity and various' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for dogmas that I've seen before include "Set of principles set down as incontrovertibly true" , "Doctrines put forward by church" , "Doctrines to be accepted without question" .)