Antidote contains a resistance for poison (6)
I believe the answer is:
curare
'for poison' is the definition.
(I've seen this before)
'antidote contains a resistance' is the wordplay.
'antidote' becomes 'cure' (I've seen this in another clue).
'contains' indicates putting letters inside.
'resistance' becomes 'r' (electronics abbreviation).
'a'+'r'='ar'
'cure' placed around 'ar' is 'CURARE'.
(Other definitions for curare that I've seen before include "Arrow poison" , "Poison put on arrows" , "Poison applied to darts" , "Poison used on darts" , "Paralysing plant poison" .)