Sailor has to achieve objective (6)
I believe the answer is:
'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).
(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" .)