Guinness of film (4)
I believe the answer is:
'guinness of film' 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" , "Guinness (a smart lad?)" , "A smart . . . . is a know-all in lace" , "Sir -- Guinness, actor" , "Smart boy?" .)