Costello's partner (6)
I believe the answer is:
abbott
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">costello's partner</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 abbott that I've seen before include "one of a comic duo" , "Costello's film partner" , "Costello's comedy partner" , "Double act partner of Lou Costello" , "Bud ... and Lou Costello" .)