Lines from last character to be found in Santa's letters (6)
I believe the answer is:
stanza
'lines' is the definition.
(group of lines in a poem)
'last character to be found in santa's letters' is the wordplay.
'last character' becomes 'Z'.
'to be found in' means one lot of letters goes inside another.
'letters' is an anagram indicator.
'santa' anagrammed gives 'stana'.
'z' placed into 'stana' is 'STANZA'.
'from' acts as a link.
(Other definitions for stanza that I've seen before include "Section of verse" , "Group of lines of verse" , "piece of poetry" , "Chunk of poetry" , "Lines of poetry forming a pattern" .)