He's not preparing the cannabis there and then (2,3,4)
I believe the answer is:
on the spot
'there and then' is the definition.
(I've seen this before)
'he's not preparing the cannabis' is the wordplay.
'preparing' is an anagram indicator (the same letters prepared differently).
'the cannabis' becomes 'pot' ('pot' can be a synonym of 'cannabis').
'hes'+'not'='hesnot'
'hesnot' anagrammed gives 'onthes'.
'onthes'+'pot'='ON-THE-SPOT'
(Other definitions for on the spot that I've seen before include "At the scene of the action" , "Ball for a penalty placed here" , "At a particular place of action" , "In difficulty" , "Snooker balls may be" .)