Sailor posted notes, 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.
'sailor posted notes' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'posted' becomes 'sent' ('send' can be a synonym of 'post').
'notes' becomes 'ee' (two E's - as in the musical note E).
'ab'+'sent'+'ee'='ABSENTEE'
'but' is the link.
Can you help me to learn more?
(Other definitions for absentee that I've seen before include "Someone not present" , "shirker?" , "He's not here!" , "are you fonder of him?" , "Deserter - truant b" .)