Carol starter (6)
I believe the answer is:
adeste
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">carol starter</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for adeste that I've seen before include "Christmas hymn: '. . . . . . fideles'" , "Christmas carol '...... fideles'" .)