Childishly silly (7)
I believe the answer is:
puerile
'childishly silly' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for puerile that I've seen before include "Childish, immature" , "Childish, silly, trivial" , "Infantile" , "Babyish" , "Childishly stupid" .)