Combine treating complex with medicine, ultimately (9)
I believe the answer is:
integrate
'combine' is the definition.
(integrating is a kind of combining)
'treating complex with medicine ultimately' is the wordplay.
'complex with' is an anagram indicator.
'ultimately' says to take the final letters.
The last letter of 'medicine' is 'e'.
'treating' with its letters anagrammed with 'e' can make 'INTEGRATE'.
(Other definitions for integrate that I've seen before include "unify" , "Combine into a whole, unite" , "Organise a get-together" , "Join" , "Incorporate into the main group" .)