It could be simple curiosity (8)
I believe the answer is:
interest
'curiosity' is the definition.
(I know that curiosity can be written as interest)
'it could be simple' is the wordplay.
I cannot really see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
'be' could be 'rest' (resting is a kind of being) and 'rest' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for interest that I've seen before include "Share in an undertaking" , "Hobby" , "Cost of borrowing" , "Condition of curiosity about amount paid for use of money" , "Gain - curiosity" .)