Apply, as influence (5)
I believe the answer is:
exert
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">apply as influence</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for exert that I've seen before include "Bring into operation" , "Bring into use, strength or influence" , "Bring into use, say strength" , "Make a great effort" , "Bring (force) to bear" .)