Sailor has to obtain an objective (6)
I believe the answer is:
target
'an objective' is the definition.
(I know that objective can be written as target)
'sailor has to obtain' is the wordplay.
'sailor' becomes 'tar' (slang term for a sailor).
'has' means one lot of letters go next to another.
'to obtain' becomes 'get' (synonyms).
'tar'+'get'='TARGET'
(Other definitions for target that I've seen before include "Object to be attained by archers (or fund raisers?)" , "Objective, mark" , "Point to be aimed at" , "Aimpoint" , "A result to aim at" .)