Giant entertains one old master (6)
I believe the answer is:
titian
'old master' is the definition.
(I know that Titian is a old master)
'giant entertains one' is the wordplay.
'giant' becomes 'titan' (titans are giant mythological creatures).
'entertains' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'titan' enclosing 'i' is 'TITIAN'.
(Other definitions for titian that I've seen before include "Sixteenth century Venetian painter" , "Painter; red tint" , "He painted" , "Venetian painter - shade of red" , "Colour named after artist" .)