Jack has to hit the mark (6)
I believe the answer is:
target
'mark' is the definition.
(synonyms)
'jack has to hit' is the wordplay.
'jack' becomes 'tar' (both are informal terms for sailor).
'has' means one lot of letters go next to another.
'to hit' becomes 'get' (getting is a kind of hitting).
'tar'+'get'='TARGET'
'the' acts as a link.
(Other definitions for target that I've seen before include "Object of attack" , "Point to be aimed at" , "A result to aim at" , "Something to aim for" , "Goal, objective" .)