I say sir - what a good advert! (3)
I believe the answer is:
gad
'i say sir' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'a good advert' is the wordplay.
'a good' becomes 'g' (abbreviation).
'advert' becomes 'ad' (synonyms).
'g'+'ad'='GAD'
'what' acts as a link.
Can you help me to learn more?
(Other definitions for gad that I've seen before include "Walk (about)" , "biblical tribe" , "carefree adventure" , "Gallivant in search of pleasure" , "wandering about" .)