The greatest dessert for a legendary caveman (3,4)
I believe the answer is:
ali baba
'a legendary caveman' is the definition.
(Ali Baba found treasure in a cave)
'the greatest dessert' is the wordplay.
'the greatest' becomes 'ali' (Muhammad Ali was known as the Greatest).
'dessert' becomes 'baba' (as in a rum baba).
'ali'+'baba'='ALI BABA'
'for' acts as a link.
(Other definitions for ali baba that I've seen before include "show" , "Chopper" , "1001 Nights hero" , "Hero from Arabian Nights who said ''open sesame''" , "Panto character against 40 thieves" .)