Starts investigating child over nasty symbol (4)
I believe the answer is:
icon
'symbol' is the definition.
(I've seen this before)
'starts investigating child over nasty' is the wordplay.
'starts' indicates taking the first letters.
The first letters of 'investigating child over nasty' is 'ICON'.
(Other definitions for icon that I've seen before include "On-screen image" , "Famous and admired person" , "A representative or sacred picture" , "Something on computer screen" , "Religious image or computer symbol" .)