Sailor sent to look up those missing (9)
I believe the answer is:
absentees
'those missing' is the definition.
I can't tell whether this defines the answer.
'sailor sent to look up' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'to look' becomes 'see'.
'up' is a reversal indicator (in down clue: letters go upwards).
'see' backwards is 'ees'.
'ab'+'sent'+'ees'='ABSENTEES'
Can you help me to learn more?
(Other definitions for absentees that I've seen before include "who may have apologised to chairman" , "They're not present at the required time" , "One gets fonder of those" .)