Childish, putting worker in case's cover (9)
I believe the answer is:
infantile
'childish' is the definition.
(I know that childish can be written as infantile)
'worker in case's cover' is the wordplay.
I cannot quite understand how this works, but
'worker' could be 'ant' (worker is a type of ant) and 'ant' is found in the answer.
'in' is within the answer.
The remaining letters 'file' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'putting' is the link.
Can you help me to learn more?
(Other definitions for infantile that I've seen before include "in early stage of development" , "Inflate in childish way" , "Babyish" , "for kids" .)