Summarise sailor's area (8)
I believe the answer is:
abstract
'summarise' is the definition.
(abstracting is a kind of summarizing)
'sailor's area' is the wordplay.
'sailor's' becomes 'ab's' (abbreviation for able seaman).
'area' becomes 'tract' (tract is a kind of area).
'abs'+'tract'='ABSTRACT'
(Other definitions for abstract that I've seen before include "Brats act in non-concrete way" , "Theoretical, not physical" , "Kandinsky painting, for example?" , "Concerned with theories rather than things" , "Summary; remove" .)