Alpha's counterpart (5)
I believe the answer is:
omega
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">alpha's counterpart</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 omega that I've seen before include "end of series" , "Ending of a series or sequence" , "Greek finalist" , "Greek long 'o'" , "16 letter" .)