Sailor dispatched to the ends of empire — one who can't be here (8)
I believe the answer is:
absentee
'one who can't be here' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'sailor dispatched to the ends of empire' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'dispatched' becomes 'sent' (I've seen this in another clue).
'to the ends of' means to remove the middle letters.
'empire' with its middle taken out is 'ee'.
'ab'+'sent'+'ee'='ABSENTEE'
Can you help me to learn more?
(Other definitions for absentee that I've seen before include "He's not here!" , "skiver" , "One is not present when required" , "one's failed to show up" , "One failing to attend" .)