Form of trestle used by writers (7)
I believe the answer is:
letters
'by writers' is the definition.
Although both the answer and definition are plural nouns, I can't understand how they can define each other.
'form of trestle' is the wordplay.
'form of' is an anagram indicator (the letters need to be in another 'form').
'trestle' anagrammed gives 'LETTERS'.
'used' is the link.
Can you help me to learn more?
(Other definitions for letters that I've seen before include "A, B, C, D etc" , "Literary culture" , "They may indicate a degree" , "Written notes" , "landlords?" .)