In Scandinavia grandpa discovered potent drug (6)
I believe the answer is:
viagra
'potent drug' is the definition.
(viagra is a drug)
'in scandinavia grandpa discovered' is the wordplay.
'in' says the answer is hidden in the clue.
'VIAGRA' can be found hidden inside 'scandinavia grandpa discovered'.
(Other definitions for viagra that I've seen before include "Drug aiding one's sex life" , "Performance-enhancing drug?" , "Male sexual aid" , "Drug treating impotence" , "potent drug?" .)