Posted a worker outside, but he's not there (8)
I believe the answer is:
absentee
'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 outside' is the wordplay.
'posted' becomes 'sent' ('send' can be a synonym of 'post').
'worker' becomes 'bee' (a worker bee).
'outside' indicates putting letters inside.
'a'+'bee'='abee'
'sent' put within 'abee' is 'ABSENTEE'.
'but' is the link.
Can you help me to learn more?
(Other definitions for absentee that I've seen before include "He's not here!" , "Deserter - truant b" , "Someone not present" , "skiver" , "One is not present when required" .)