Irritate, fester (6)
I believe the answer is:
rankle
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">irritate</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that rankle is a more specific form of the action irritate)</span><br/ ><br/ >'<span class="explanation-format__original">fester</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for rankle that I've seen before include "Continue to vex" , "Go on irritating" , "keep hurting" , "Cause lasting bitterness and resentment" , "Continue to cause annoyance or bitterness" .)