Set on reversing unpleasant situation: think about that (11)
I believe the answer is:
superimpose
'set' is the definition.
(superimposing is a kind of setting)
'unpleasant situation think about that' is the wordplay.
'unpleasant situation' becomes 'erim' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'think' becomes 'suppose' (synonyms).
'about that' is an insertion indicator.
'erim' going inside 'suppose' is 'SUPERIMPOSE'.
'on reversing' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for superimpose that I've seen before include "Overlay" , "Place one over another" , "Place something over something else" .)