Powerful dope hidden in bed (6)
I believe the answer is:
cogent
'powerful' is the definition.
(thesaurus)
'dope hidden in bed' is the wordplay.
'dope' becomes 'gen' (I've seen this in other clues).
'hidden in' is an insertion indicator.
'bed' becomes 'cot' (cot is a kind of bed).
'gen' put into 'cot' is 'COGENT'.
(Other definitions for cogent that I've seen before include "'Clear and logical, argument say (6)'" , "'Powerfully persuasive, argument say (6)'" , "Forceful and to the point" , "Compelling (argument)" , "Compelling or convincing" .)