Fair, therefore bang on (4,2)
I believe the answer is:
just so
'bang on' is the definition.
'fair therefore' is the wordplay.
'fair' becomes 'just' ('just' can be a synonym of 'fair'**).
'therefore' becomes 'so'.
'just'+'so'='JUST SO'
(Other definitions for just so that I've seen before include "Carefully planned" , "I agree!" , "how some stories were described" , "In a careful manner" , "Like Kipling's stories" .)