Unusual spite shown by the French in a letter (7)
I believe the answer is:
epistle
'letter' is the definition.
(I know that epistle is a type of letter)
'unusual spite shown by the french' is the wordplay.
'unusual' is an anagram indicator.
'shown by' is a charade indicator (letters next to each other).
'the french' becomes 'le' ('the' in French).
'spite' anagrammed gives 'epist'.
'epist'+'le'='EPISTLE'
'in a' is the link.
(Other definitions for epistle that I've seen before include "screed" , "Biblical missive" , "Apostolic letter" , "verse composition" , "Missive; letter" .)