Witness observed children's plaything (6)
I believe the answer is:
seesaw
'children's plaything' is the definition.
'witness observed' is the wordplay.
'witness' becomes 'see' (synonyms).
'observed' becomes 'saw' (I've seen this before).
'see'+'saw'='SEESAW'
(Other definitions for seesaw that I've seen before include "Playground rocker" , "Pivoted amusement" , "wild mare" , "swing" , "Fluctuating playground ride" .)