Time to rule heartlessly — destroy! (9)
I believe the answer is:
eradicate
'destroy' is the definition.
(to eradicate is to destroy or eliminate)
'time to rule heartlessly' is the wordplay.
'time' becomes 'era' (an era is a period of time).
'to rule heartlessly' becomes 'dicate' (I am not sure about this - if you are sure you should believe this answer much more).
'era'+'dicate'='ERADICATE'
Can you help me to learn more?
(Other definitions for eradicate that I've seen before include "Get rid of, remove all traces" , "Expunge" , "carry out a purge" , "Destroy completely" , "Excise" .)