Main article on redesigned trailer (8)
I believe the answer is:
arterial
'main' is the definition.
Although both the answer and definition are adjectives, I cannot see how they can define each other.
'article on redesigned trailer' is the wordplay.
'article' becomes 'a' (the indefinite article in English).
'on' means one lot of letters goes inside another.
'redesigned' is an anagram indicator.
'trailer' is an anagram of 'arteril'.
'a' going inside 'arteril' is 'ARTERIAL'.
Can you help me to learn more?
(Other definitions for arterial that I've seen before include "Road that connects towns or cities" , "Like certain vessels" , "Relating to the major blood vessels" , "Major route with minor branches" , "Relating to major routes" .)