It's perplexing - be thinking things over (8)
I believe the answer is:
bemusing
'it's perplexing' is the definition.
I can't tell whether this defines the answer.
'be thinking things over' is the wordplay.
'be thinking' becomes 'musing' (I've seen this before).
'things' becomes 'be' (I can't explain this - if you can you should give a lot more credence to this answer).
'over' means one lot of letters go next to another.
'musing' put after 'be' is 'BEMUSING'.
Can you help me to learn more?
(Other definitions for bemusing that I've seen before include "puzzling" , "Like a puzzle" .)