Fictional Uncle (5)
I believe the answer is:
remus
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fictional uncle</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 remus that I've seen before include "Co-founder of Rome" , "US relative who did accounts" , "Brother of 15" , "Famous twin" , "murdered brother?" .)