Powerful drug supported by hospital department (6)
I believe the answer is:
potent
'powerful' is the definition.
('potent' can be similar in meaning to 'powerful')
'drug supported by hospital department' is the wordplay.
'drug' becomes 'pot' (slang term for cannabis).
'supported by' says to put letters next to each other (letters go below - in down clue).
'hospital department' becomes 'ent'.
'pot'+'ent'='POTENT'
(Other definitions for potent that I've seen before include "Strong or concentrated, like some drink" , "Top Ten (anag.)" , "Strong in operation" , "Forceful" , "Intoxicating" .)