A sailors land is not concrete (8)
I believe the answer is:
abstract
'is not concrete' is the definition.
('abstract' is the opposite to 'concrete')
'a sailors land' is the wordplay.
'a sailors' becomes 'abs' (AB is short for able seaman).
'land' becomes 'tract' (I've seen this in other clues).
'abs'+'tract'='ABSTRACT'
(Other definitions for abstract that I've seen before include "Theoretical; painting" , "Appropriate - theoretical - summary" , "Brats act in non-concrete way" , "Take away - theoretical" , "Conceptual" .)