Childishly sulky (8)
I believe the answer is:
petulant
'childishly sulky' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for petulant that I've seen before include "Pouting" , "Fractious, pouty" , "peevish impatience" , "Sullen in a peevish way" , "Childishly sulky or bad-tempered" .)