Mark in a compound word (6)
I believe the answer is:
hyphen
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mark in a compound word</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for hyphen that I've seen before include "Dash breaking word" , "Stroke between syllables or words" , "Sign linking words" , "It joins two words" , "Mark joining two words" .)