Livestock here help, so it's said, to supply a drug dealer (10)
I believe the answer is:
Here is my best explanation:
'drug dealer' is the definition.
(I've seen this before)
'livestock here help so it's said to supply a' is the subsidiary indication.
I cannot really see how this works, but
'supply' could be 'arm' (arming is a kind of supplying) and 'arm' is present in the answer.
'a' is present in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
This explanation may well be incorrect...
(Other definitions for pharmacist that I've seen before include "Drugs dispenser", "Drug dispenser", "Dispenser of drugs", "Drug dealer", "Medicine dispenser", "Chemist" and "High-street chemist".)