Excerpt from sailor's treatise (8)
I believe the answer is:
abstract
'excerpt' is the definition.
(abstract can mean to extract or excerpt)
'sailor's treatise' is the wordplay.
'sailor's' becomes 'ab's' (abbreviation for able seaman).
'treatise' becomes 'tract' (tract is a kind of treatise).
'abs'+'tract'='ABSTRACT'
'from' acts as a link.
(Other definitions for abstract that I've seen before include "Appropriate - theoretical - summary" , "profound" , "a work of modern art" , "Steal" , "Theoretical; abstruse" .)