Strong effect (6)
I believe the answer is:
impact
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">strong effect</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 impact that I've seen before include "Forceful striking" , "Marked effect; hit" , "Repercussion" , "Result" , "Forceful contact between two objects" .)