One missing a worker sent inside (8)
I believe the answer is:
absentee
'one missing' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a worker sent inside' is the wordplay.
'worker' becomes 'bee' (a worker bee).
'inside' indicates putting letters inside.
'bee' placed around 'sent' is 'bsentee'.
'a'+'bsentee'='ABSENTEE'
Can you help me to learn more?
(Other definitions for absentee that I've seen before include "One failing to attend" , "are you fonder of him?" , "skiver" , "to be called in? No!" , "Person who should be present but isn't" .)