Preserve a mischievous child (6)
I believe the answer is:
pickle
This clue might be a double definition.
'preserve' is the first definition.
(pickling is a kind of preserving)
'a mischievous child' is the second definition.
'pickle' can be an answer for 'child' (I have seen 'Troublesome child' mean 'pickle' so perhaps 'child' could also mean 'pickle'). I'm not certain of the 'a mischievous' bit.
Can you help me to learn more?
(Other definitions for pickle that I've seen before include "A preserved vegetable or a fine mess" , "Lather - relish" , "Sticky situation" , "Preserve in liquid" , "Awkward situation" .)