Chief in importance (4)
I believe the answer is:
main
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">chief in importance</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for main that I've seen before include "Chief; sea; pipe" , "Source of electricity" , "Cardinal" , "Chief - principal electricity cable" , "Ocean or principal pipe" .)