Sailor understands objectives (7)
I believe the answer is:
targets
'objectives' is the definition.
('target' can be a synonym of 'objective')
'sailor understands' is the wordplay.
'sailor' becomes 'tar' (tar is a kind of sailor).
'understands' becomes 'gets' (getting is a kind of understanding).
'tar'+'gets'='TARGETS'
(Other definitions for targets that I've seen before include "Goals" , "Butts" , "Reference points to shoot at" , "Aim points" , "objectives of firing people" .)