Chief idea in article I endorsed (5)
I believe the answer is:
theme
'chief idea' is the definition.
'theme' can be an answer for 'idea' (theme is a kind of idea). I am not sure about the 'chief' bit.
'article i endorsed' is the wordplay.
I cannot really see how this works, but
'article' could be 'the' (the definite article in English) and 'the' is found in the answer.
'i' could be 'me' and 'me' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'in' acts as a link.
Can you help me to learn more?
(Other definitions for theme that I've seen before include "Burden" , "Subject matter, motif" , "basis for variations" , "Subject of a talk or piece of writing" , "Unifying motif" .)