One child's head got stuck in it some time back (4)
I believe the answer is:
once
'some time back' is the definition.
(I've seen this in another clue)
'one child's head got stuck in it' is the wordplay.
'head' indicates taking the first letters.
'got stuck' becomes 'n' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
The initial letters of 'one childs' is 'oc'.
'oc' enclosing 'n' is 'onc'.
'onc'+'e'='ONCE'
Can you help me to learn more?
(Other definitions for once that I've seen before include "Formerly; as soon as" , "Previously" , "in bygone era" , "A single time" , "Formerly, at one time" .)