Medicinal powder left over in kind of cigarette (7)
I believe the answer is:
calomel
'medicinal powder' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'left over in kind of cigarette' is the wordplay.
'left' becomes 'l' (common abbreviation).
'over' becomes 'o' (cricket abbreviation).
'in' indicates putting letters inside.
'kind of cigarette' becomes 'camel' (I can't explain this - if you can you should give a lot more credence to this answer).
'l'+'o'='lo'
'lo' put within 'camel' is 'CALOMEL'.
Can you help me to learn more?
(Another definition for calomel that I've seen is " All come (anag) - purgative".)