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