Let it alter heading (5)
I believe the answer is:
title
'heading' is the definition.
(I know that title is a type of heading)
'let it alter' is the wordplay.
'alter' indicates an anagram.
'let'+'it'='letit'
'letit' anagrammed gives '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" .)