Agriculturist reportedly beheaded Green: it's to do with drugs (15)

'to do with drugs' is the definition.

'agriculturist reportedly beheaded green' is the wordplay.
'agriculturist' becomes 'farmer' (I've seen this before).
'reportedly' indicates a 'sounds like' (homophone) clue.
'beheaded' means to remove the first letter (the head of the word is removed).
'green' becomes 'ecological' (I've seen this in another clue).
'farmer' sounds like 'pharma'.
'ecological' with its initial letter taken off is 'cological'.
'pharma'+'cological'='PHARMACOLOGICAL'

'it's' acts as a link.