Runs work a very long time with setback (8)
I believe the answer is:
operates
'runs work' is the definition.
'operates' can be an answer for 'runs' ('operate' can be a synonym of 'run'). I am unsure of the 'work' bit.
'a very long time with setback' is the wordplay.
'a very long time' becomes 'opera' (I can't justify this - if you can you should believe this answer much more).
'with' means one lot of letters go next to another.
'setback' becomes 'tes' (I've seen this in other clues).
'opera'+'tes'='OPERATES'
Can you help me to learn more?
(Other definitions for operates that I've seen before include "Functions" , "'Uses, manipulates (8)'" , "Acts" , "Performs sugery" , "Directs or controls, machinery say" .)