The French race native to Latvia! (4)
I believe the answer is:
'native to latvia' is the definition.
I can't judge whether this definition defines the answer.
'the french race' is the wordplay.
'the french' becomes 'le' ('the' in French).
'race' becomes 'tt'.
Can you help me to learn more?
(Other definitions for lett that I've seen before include "Baltic citizen" , "northerner" , "East European" , "Balt" , "Native of Latvia" .)