Make heavy demands on (3)
I believe the answer is:
tax
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">make heavy demands on</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 tax that I've seen before include "Levy; make demands on" , "Levy for public purposes" , "Be trying" , "Strain; levy" , "Government levy" .)