Sailor has to achieve objective (6)
I believe the answer is:
target
'objective' is the definition.
(I know that objective can be written as target)
'sailor has to achieve' is the wordplay.
'sailor' becomes 'tar' (slang term for a sailor).
'has' means one lot of letters go next to another.
'to achieve' becomes 'get' (I've seen this in another clue).
'tar'+'get'='TARGET'
(Other definitions for target that I've seen before include "Point to be aimed at" , "Thing to aim at" , "Mark to be aimed at" , "Mark to aim at" , "desired objective" .)