Consistent with a letter (9)
I believe the answer is:
consonant
I believe this clue is a double definition.
'consistent with' is the first definition.
(synonyms)
'a letter' is the second definition.
(I know that consonant is a type of letter)
(Other definitions for consonant that I've seen before include "B, C, D or F, perhaps?" , "Agreeing" , "Letter; harmonious chord" , "One of several characters" , "In agreement" .)