Migraine (8)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">migraine</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for headache that I've seen before include "Informally a worrying problem - take an asprin" , "A pain in the brain" , "Migraine" , "nasty problem" , "Cause of worry/trouble" .)