Letter pile set to be rearranged (7)
I believe the answer is:
epistle
'letter' is the definition.
(epistle is a kind of letter)
'pile set to be rearranged' is the wordplay.
'to be rearranged' is an anagram indicator.
'pile'+'set'='pileset'
'pileset' anagrammed gives 'EPISTLE'.
(Other definitions for epistle that I've seen before include "Apostle's letter" , "may read this" , "Literary work" , "(E.g. Paul's) letter" , "A letter - as from Apostle?" .)