Only even prime number (3)
I believe the answer is:
two
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">only even prime number</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 two that I've seen before include "Number in company, some say" , "eg Twins" , "Minimal number of partners" , "A low number" , "Number of Gentlemen of Verona" .)