Costello's partner (6)
I believe the answer is:
abbott
'costello's partner' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)