Sailor to understand objective (6)
I believe the answer is:
target
'objective' is the definition.
('target' can be a synonym of 'objective')
'sailor to understand' is the wordplay.
'sailor' becomes 'tar' (tar is a kind of sailor).
'to understand' becomes 'get' (getting is a kind of understanding).
'tar'+'get'='TARGET'
(Other definitions for target that I've seen before include "Point to aim at (6)." , "Goal, objective" , "Aim of an attack" , "Thing aimed at" , "Aim, ambition" .)