One gets beneath the surface of cannabis, uncovering disease (8)
I believe the answer is:
potholer
'one gets beneath the surface' is the definition.
'cannabis uncovering disease' is the wordplay.
'cannabis' becomes 'pot' ('pot' can be a synonym of 'cannabis').
'uncovering' says to take the centre.
'disease' becomes 'cholera' (cholera is a kind of disease).
The central letters of 'cholera' are 'holer'.
'pot'+'holer'='POTHOLER'
'of' acts as a link.
(Other definitions for potholer that I've seen before include "Speleologist" , "Explorer of underground caves" , "Caver" .)