Surprise attacker (6)
I believe the answer is:
raider
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">surprise attacker</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 raider that I've seen before include "One who makes a sudden swift attack" , "Maker of surprise attack" , "person who's involved in sacking?" , "Viking" , "person taking food from larder" .)