The sort of letters one finds in a crossword (7)
I believe the answer is:
anagram
This clue is a double definition.
'the sort of letters' is the first definition.
(I've seen this before)
'one finds in a crossword' is the second definition.
(crosswords often use anagrams)
(Other definitions for anagram that I've seen before include "Word(s) rearranged to form others" , "type of clue" , "Gelding can be niggled by this" , "Argue the point" , "Word play" .)