After you send it (4)
I believe the answer is:
post
'after' is the definition.
(prefix meaning after, eg post-war)
'you send it' is the wordplay.
I cannot quite see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
The remaining letters 'pos' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for post that I've seen before include "delivery" , "may be winning" , "Publish' mail' job" , "Member < to comment on website" , "Job - letters - stake in ground" .)