Powerful drug essential for slimming (6)
I believe the answer is:
potent
'powerful' is the definition.
(I know that powerful can be written as potent)
'drug essential for slimming' is the wordplay.
I cannot really see how this works, but
'drug' could be 'pot' (slang term for cannabis) and 'pot' is found within 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 potent that I've seen before include "Having great power" , "Forceful" , "Top Ten (anag.)" , "Strong in operation" , "Powerful or effective" .)