After a short time, get hold of small items put in the post (5,4)

'after a' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'short time get hold of small items put in the post' is the wordplay.
I cannot really understand how this works, but
'get' could be 'bag' (bag can mean to secure or obtain) and 'bag' is present in the answer.
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is located in the answer.
'short time' could be 'jiffy' (jiffy is a kind of short time) and 'jiffy' is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?