Sir Guinness (4)
I believe the answer is:
alec
'sir guinness' 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 "Sir -- Guinness, actor" , "Sir - Douglas-Home, former PM" , "Smart chap" , "Guinness (a smart lad?)" , "Smart boy?" .)