Upset the writer receiving a poor mark in test (4)
I believe the answer is:
exam
'test' is the definition.
(I know that test can be written as exam)
'upset the writer receiving a poor mark in' is the wordplay.
'upset' says the letters should be written backwards.
'the writer' becomes 'me' (the person who wrote the clues).
'receiving' means one lot of letters goes inside another.
'poor mark in' becomes 'x' (I am not sure about this - if you are sure you should believe this answer much more).
'a'+'x'='ax'
'me' going around 'ax' is 'maxe'.
'maxe' written backwards gives 'EXAM'.
Can you help me to learn more?
(Other definitions for exam that I've seen before include "paper" , "Test (abbrev.)" , "Set of questions evaluating skill or knowledge, in short" , "Students sit it" , "In brief, a test as in school" .)