Sweet bad stuck in gum (8)
I believe the answer is:
pastille
'sweet' is the definition.
(pastille is a kind of sweet)
'bad stuck in gum' is the wordplay.
'bad' becomes 'ill' (associated in meaning).
'stuck in' is an insertion indicator.
'gum' becomes 'paste' (I've seen this before).
'ill' going inside 'paste' is 'PASTILLE'.
(Other definitions for pastille that I've seen before include "Flavoured or medicated lozenge" , "tablet" , "something for a sore throat" , "Small sweet/lozenge" , "Cough lozenge" .)