A bomb that's sweet (4)
I believe the answer is:
mint
I believe this is a double definition.
'a bomb' is the first definition.
(I've seen this before)
'sweet' is the second definition.
(mint is a kind of sweet)
'that's' acts as a link.
(Other definitions for mint that I've seen before include "and 18 Down: Southern US cocktail" , "Plant - in perfect condition" , "Flavour of the money-maker?" , "Coin-making place or nice lamb sauce" , "strike?" .)