Post given to short nob regarding PM (9)
I believe the answer is:
afternoon
'pm' is the definition.
(I've seen this in another clue)
'post given to short nob regarding' is the wordplay.
'post' becomes 'after' (the prefix 'post-' means 'after').
'given to' says to put letters next to each other.
'short' means to remove the last letter.
'regarding' becomes 'on' (on or regarding a particular topic).
'nob' with its last letter removed is 'no'.
'after'+'no'+'on'='AFTERNOON'
(Other definitions for afternoon that I've seen before include "half a day?" , "Post-midday" , "PM includes this" , "Time of day" , "Then it's early in this!" .)