Medicinal substance left by your medical officer (6)
I believe the answer is:
thymol
'medicinal substance' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'left by your medical officer' is the wordplay.
'left' becomes 'l' (common abbreviation).
'by' is a charade indicator (letters next to each other).
'your' becomes 'thy' (I've seen this before).
'medical officer' becomes 'mo' (this could be a standard abbreviation of which I'm unaware).
'thy'+'mo'='thymo'
'l' after 'thymo' is 'THYMOL'.
Can you help me to learn more?
(Another definition for thymol that I've seen is " chemical antiseptic".)