Latvian ruler's character (6)
I believe the answer is:
letter
'character' is the definition.
(letter is a kind of character)
'latvian ruler's' is the wordplay.
'latvian' becomes 'lett'.
'ruler' becomes 'er' (abbreviation for Elizabeth Regina).
'lett'+'er'='LETTER'
(Other definitions for letter that I've seen before include "Alphabetic symbol" , "Form of correspondence" , "A, B or C, for example" , "Written message" , "Piece of correspondence" .)