Drug provides alternative in medical service (5)
I believe the answer is:
horse
'drug' is the definition.
(horse is a slang term for heroin)
'alternative in medical service' is the wordplay.
I cannot really understand how this works, but
'alternative' could be 'or' and 'or' is present in the answer.
This explanation may well be incorrect...
'provides' acts as a link.
Can you help me to learn more?
(Other definitions for horse that I've seen before include "Nag, hack" , "runner" , "bay, perhaps" , "Vaulting --; -- sense" , "Shore (anag)" .)