Sleep it off is the message (7)
I believe the answer is:
epistle
'the message' is the definition.
(thesaurus)
'sleep it off' is the wordplay.
'off' indicates an anagram.
'sleep'+'it'='sleepit'
'sleepit' anagrammed gives 'EPISTLE'.
'is' is the link.
(Other definitions for epistle that I've seen before include "one from Paul?" , "extract of the Bible" , "A letter - as from Apostle?" , "(E.g. Paul's) letter" , "Romans perhaps" .)