Letter P among terrible tiles at end of Scrabble (7)
I believe the answer is:
epistle
'letter' is the definition.
(I know that epistle is a type of letter)
'p among terrible tiles at end of scrabble' is the wordplay.
'among' means one lot of letters goes inside another (inserted letters are placed among others).
'terrible' indicates an anagram.
'at end of' indicates one should take the final letters.
The final letter of 'scrabble' is 'e'.
'tiles' with letters rearranged gives 'eistl'.
'p' placed within 'eistl' is 'epistl'.
'epistl'+'e'='EPISTLE'
(Other definitions for epistle that I've seen before include "a for instance" , "screed" , "may be written off" , "extract of the Bible" , "Pets lie (anag)" .)