Pat's mate turned the old Scandinavian's spiked drink (6,4)
I believe the answer is:
mickey finn
'spiked drink' is the definition.
(I've seen this before)
'pat's mate turned the old scandinavian's' is the wordplay.
'pat's mate' becomes 'mick' (I can't explain this - if you can you should give a lot more credence to this answer).
'turned' shows that the letters should be reversed in order.
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'scandinavian' becomes 'finn' (I've seen this before).
'ye' written backwards gives 'ey'.
'mick'+'ey'+'finn'='MICKEY FINN'
Can you help me to learn more?
(Other definitions for mickey finn that I've seen before include "spiked drink" , "knockout" , "Doped drink" .)