Commonly used pain-killer (7)
I believe the answer is:
aspirin
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">commonly used pain-killer</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 aspirin that I've seen before include "Pain-relieving tablet" , "Headache killer?" , "Common drug in Paris" , "headache cure" , "Headache pill" .)