Wreak havoc over drug being less potent (6)
I believe the answer is:
weaker
'less potent' is the definition.
(potent means strong)
'wreak havoc over drug' is the wordplay.
'havoc' is an anagram indicator (I've seen this in other clues).
'over' means one lot of letters goes inside another.
'drug' becomes 'e' (E is another name for ecstasy).
'wreak' anagrammed gives 'weakr'.
'weakr' enclosing 'e' is 'WEAKER'.
'being' is the link.
(Other definitions for weaker that I've seen before include "Less powerful" , "More lacking in strength" , "More powerless" , "Wanting in strength" , "More feeble or frail" .)