No-show sailors put on course right away (8)
I believe the answer is:
absentee
'no-show' is the definition.
(person who has not shown up for something)
'sailors put on course right away' is the wordplay.
'sailors' becomes 'abs' (AB is short for able seaman).
'put' becomes 'tee' (teeing is a kind of putting).
'on' says to put letters next to each other.
'course right away' becomes 'en' (I can't explain this - if you can you should give a lot more credence to this answer).
'tee' after 'en' is 'entee'.
'abs'+'entee'='ABSENTEE'
(Other definitions for absentee that I've seen before include "One who should be but isn't present" , "Missing person" , "Person who should be present but isn't" , "One away from work, etc" , "to prompt a hearty increase in affection!" .)