Intensive attack (5)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">intensive attack</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 blitz that I've seen before include "Sustained bombing attack" , "Sustained bombing as in WWII" , "Sudden fierce attack" , "Sustained attack (from the air?)" , "Intensive military attack" .)