Principal, prime (4)
I believe the answer is:
main
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">principal</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(both can mean major)</span><br/ ><br/ >'<span class="explanation-format__original">prime</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for main that I've seen before include "location for 9 [ARMADA]" , "(The mighty) sea" , "one might expect tap water from it" , "Most important or greatest in size" , "Most important element" .)