Greek a (5)
I believe the answer is:
alpha
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">greek a</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 alpha that I've seen before include "piece of Greek writing" , "European character" , "Foremost" , "Most prominent star" , "starter" .)