Ultimately this medication left unexpected consequences (9)
I believe the answer is:
spillover
'ultimately this medication left unexpected consequences' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'ultimately this medication left unexpected consequences' is the wordplay.
'ultimately' says to take the final letters.
'medication' becomes 'pill' (pill is a kind of medication).
'left unexpected consequences' becomes 'over' (I can't justify this - if you can you should give a lot more credence to this answer).
The final letter of 'this' is 's'.
's'+'pill'+'over'='SPILLOVER'
Can you help me to learn more?
(Other definitions for spillover that I've seen before include "excess that's spread around" , "could be too much to cope with" , "unexpected consequence" .)