Strong marijuana - about half a score (6)
I believe the answer is:
potent
'strong' is the definition.
('potent' can be a synonym of 'strong')
'marijuana about half a score' is the wordplay.
'marijuana' becomes 'pot' ('pot' can be a synonym of 'marijuana').
'about' indicates putting letters inside.
'half a score' becomes 'ten' (I've seen this before).
'pot' going around 'ten' is 'POTENT'.
(Other definitions for potent that I've seen before include "Powerful or effective" , "Top Ten (anag.)" , "Powerful, influential" , "Forceful" , "Strong or concentrated, like some drink" .)