So then got hooked by the drug immediately (2,3,4)
I believe the answer is:
on the spot
'immediately' is the definition.
(I've seen this before)
'so then got hooked by the drug' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'then' is 'nthe' which is present in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for on the spot that I've seen before include "Where face cream may be applied" , "Like a shot" , "Challenged" , "Where cream may go" , "Present at an event" .)