Strong or effective (6)
I believe the answer is:
potent
I believe this clue is a double definition.
'strong' is the first definition.
(synonyms)
'effective' is the second definition.
(associated in meaning)
(Other definitions for potent that I've seen before include "Mighty" , "Strong or effective" , "Having great power" , "Concentrated or strong, like liquor" , "Powerful or effective" .)