Prescription amount (4)
I believe the answer is:
dose
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">prescription amount</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for dose that I've seen before include "Prescribed amount of medicine" , "Measured quantity of medicine" , "Measure of medicine" , "some Viagra, perhaps?" , "Measured amount of medicine" .)