Wallop in the tummy? (4)
I believe the answer is:
beer
'wallop in the tummy?' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
This is the entire clue.
Can you help me to learn more?
(Other definitions for beer that I've seen before include "Hop drink" , "See 4 down" , "Fermented malt and hops" , ""Give me a woman who loves __ and I will conquer the world" (Kaiser Wilhelm)" , "It may be 13 across-ed!" .)