Writes one's first letters (8)
I believe the answer is:
initials
'first letters' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for initials that I've seen before include "Signs briefly" , "First letters as of names" , "Components of an acronym" , "Lisa in it (anag)" .)