Ineffectual American drug? Not so (7)
I believe the answer is:
useless
'ineffectual' is the definition.
(similar in meaning)
'american drug? not so' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'drug?' becomes 'e' (E is another name for ecstasy).
'not so' becomes 'less' (I've seen this in another clue).
'us'+'e'+'less'='USELESS'
(Other definitions for useless that I've seen before include "Fruitless" , "Of no practical or worthwhile application" , "idle" , "Is Sue less impractical?" , "Ineffectual" .)