Knighted actor Guinness (4)
I believe the answer is:
alec
'knighted actor guinness' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for alec that I've seen before include "Actor Guinness is smart" , "Smart man?" , "Smart boy?" , "Smart ..., he thinks he knows everything" , "Boy's name (a smart one?)" .)