Strong drug on hospital department (6)
I believe the answer is:
potent
'strong' is the definition.
('potent' can be a synonym of 'strong')
'drug on hospital department' is the wordplay.
'drug' becomes 'pot' (slang term for cannabis).
'on' says to put letters next to each other.
'hospital department' becomes 'ent'.
'pot'+'ent'='POTENT'
(Other definitions for potent that I've seen before include "Powerful, influential" , "Powerful or effective" , "Mighty" , "Strong or effective" , "Intoxicating" .)