Let it alter heading (5)
I believe the answer is:
title
'heading' is the definition.
(title is a kind of heading)
'let it alter' is the wordplay.
'alter' indicates anagramming the letters.
'let'+'it'='letit'
'letit' is an anagram of 'TITLE'.
(Other definitions for title that I've seen before include "Name of book or lord" , "Name of book, picture or rank" , "Lord, sir, doctor etc." , "Right - name - rank" , "Legal document regarding transfer of ownership: appellation" .)