Small child's great power reported (4)
I believe the answer is:
mite
'small child's' is the definition.
(informal term)
'great power reported' is the wordplay.
'great power' becomes 'might' (I've seen this before).
'reported' indicates a 'sounds like' (homophone) clue.
'might' sounds like 'MITE'.
(Other definitions for mite that I've seen before include "Arachnid; small child" , "Tiny arachnid, person or amount of money" , "Very small creature or child" , "Tiny contribution, child" , "Minute arachnid" .)