Child’s play, this innings from one used to handling deliveries? (8,5)
I believe the answer is:
postmans knock
'child's play this' is the definition.
(postman's knock is a children's game)
'innings from one used to handling deliveries?' is the wordplay.
I cannot quite see how this works, but
'one' could be 'an' (I've seen this in other clues) and 'an' is found within the answer.
This explanation may well be incorrect...
(Other definitions for postmans knock that I've seen before include "that may lead to smacking?" , "sometimes followed by Pass the Parcel?" , "Party game where a kiss is exchanged for a pretend letter" , "One of 1 down 19 down" , "Osculatory party game" .)