Irregular fighter (9)
I believe the answer is:
guerrilla
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">irregular fighter</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 guerrilla that I've seen before include "political insurgent" , "Combatant" , "Person who fights as one of small independent group" , "Underground fighter" , "Irregular bandit" .)