Greeting PM? (3,4,2,3)
I believe the answer is:
the time of day
'greeting pm?' is the definition.
The answer and definition can be both related to time as well as being singular nouns.
Maybe there's a link between them I don't understand?
This is the entire clue.
Can you help me to learn more?
(Other definitions for the time of day that I've seen before include "Hand-held computer" , "It is passed in conversation" .)