His gag about a dish (6)
I believe the answer is:
haggis
'a dish' is the definition.
(haggis is a kind of dish)
'his gag about' is the wordplay.
'about' indicates an anagram.
'his'+'gag'='hisgag'
'hisgag' anagrammed gives 'HAGGIS'.
(Other definitions for haggis that I've seen before include "Scottish savoury combining lamb offal and oatmeal" , "Dish served piping" , "Scottish offal dish" , "Burns's "Great chieftain o' the puddin'-race"" , "Scottish grub" .)