Beer and cold Guinness, say (4)
I believe the answer is:
alec
'guinness say' is the definition.
(actor Alec Guinness, 1914-2000)
'beer and cold' is the wordplay.
'beer' becomes 'ale' (type of beer).
'and' means one lot of letters go next to another.
'cold' becomes 'C' (abbreviation, eg on taps).
'ale'+'c'='ALEC'
(Other definitions for alec that I've seen before include "Sir - Douglas-Home, former PM" , "Sir -- Guinness, actor" , "Smart man?" , "Guinness (a smart lad?)" , "Boy's name (a smart one?)" .)