Posted a worker on guard, but he's not there (8)
I believe the answer is:
absentee
'guard but he's not there' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'posted a worker on' is the wordplay.
'posted' becomes 'sent' ('send' can be a synonym of 'post').
'worker' becomes 'bee' (a worker bee).
'on' is an insertion indicator (as in clothing 'on' a person).
'a'+'bee'='abee'
'sent' placed within 'abee' is 'ABSENTEE'.
Can you help me to learn more?
(Other definitions for absentee that I've seen before include "Missing person" , "shirker?" , "One who stays away" , "One is not present when required" , "to be called in? No!" .)