Cocktail one can spoil, layering parts from the bottom (7)
I believe the answer is:
martini
'cocktail' is the definition.
(I know that Martini is a type of cocktail)
'spoil layering parts from the bottom' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'layering parts' becomes 'init' (I can't explain this - if you can you should give a lot more credence to this answer).
'from the bottom' is a reversal indicator (in a down clue, letters come from the bottom).
'init' written backwards gives 'tini'.
'mar'+'tini'='MARTINI'
'one can' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for martini that I've seen before include "Alternative name for Simone Memmi, pre-Renaissance Italian painter" , "rifle" , "obtained from a bottle" , "Italian drink" , "Sienese painter" .)