Sailor almost finished in the crow's nest? (5)
I believe the answer is:
above
'in the crow's nest?' is the definition.
(the crow's nest is up high on a ship)
'sailor almost finished' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'almost' means to remove the last letter (most of the word but not all of it).
'finished' becomes 'over' (similar in meaning).
'over' with its last letter taken off is 'ove'.
'ab'+'ove'='ABOVE'
(Other definitions for above that I've seen before include "Superior to" , "Overhead" , "In preference to" , "Aforementioned" , "Mentioned earlier" .)