Note changed to indicate empty line (4)
I believe the answer is:
tone
'empty line' is the definition.
'tone' can be an answer for 'line' (thesaurus). I'm not sure about the 'empty' bit.
'note changed' is the wordplay.
'changed' is an anagram indicator.
'note' with letters rearranged gives 'TONE'.
'to indicate' is the link.
Can you help me to learn more?
(Other definitions for tone that I've seen before include "accent" , "feel" , "Colour quality; note" , "Shade" , "Pitch of a voice" .)