Is sweet, but can make you sick in a sticky coating (8)
I believe the answer is:
pastille
'is sweet' is the definition.
(I've seen this before)
'sick in a sticky coating' is the wordplay.
'sick' becomes 'ill' (similar in meaning).
'in' is an insertion indicator.
'a sticky coating' becomes 'paste' (I can't explain this - if you can you should give a lot more credence to this answer).
'ill' placed inside 'paste' is 'PASTILLE'.
'but can make you' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for pastille that I've seen before include "Suck this" , "Lozenge, small sweet for sucking" , "something for a sore throat" , "Flavoured or medicated lozenge" , "One medicated sweet is the alternative" .)