Epidemic requires change, taking steps (10)
I believe the answer is:
pestilence
'epidemic' is the definition.
(I've seen this before)
'change taking steps' is the wordplay.
'change' becomes 'pence' (I have seen 'Small change ' mean 'pence' so perhaps 'change' could also mean 'pence').
'taking' means one lot of letters goes inside another.
'steps' becomes 'stile' (I've seen this in other clues).
'pence' going around 'stile' is 'PESTILENCE'.
'requires' is the link.
(Other definitions for pestilence that I've seen before include "evil force" , "Fatal epidemic disease" , "it's an agent of evil" , "Epidemic disease with a high death rate" , "morally harmful influence" .)