Guinness of "The Ladykillers" (4)
I believe the answer is:
alec
'guinness of the ladykillers' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for alec that I've seen before include "A smart . . . . is a know-all in lace" , "Sir - Douglas-Home, former PM" , "Smart ..., he thinks he knows everything" , "Actor Guinness is smart" , "Boy's name (a smart one?)" .)