Unseemly spite shown by the Parisian in long letter (7)
I believe the answer is:
epistle
'long letter' is the definition.
(I've seen this before)
'unseemly spite shown by the parisian' is the wordplay.
'unseemly' indicates an anagram.
'shown by' means one lot of letters go next to another.
'the parisian' becomes 'le' (definite article in French).
'spite' anagrammed gives 'epist'.
'epist'+'le'='EPISTLE'
'in' acts as a link.
(Other definitions for epistle that I've seen before include "A letter from St Paul" , "Paul's work" , "Apostle's letter" , "may read this" , "one from Paul?" .)