Back on route to Hull initially — not south! (5)
I believe the answer is:
north
'not south' is the definition.
('north' is the opposite to 'south')
'back on route to hull initially' is the wordplay.
'back' shows that the letters should be reversed in order.
'initially' says to take the initial letters.
The first letters of 'route to hull' is 'rth'.
'on' back-to-front is 'no'.
'no'+'rth'='NORTH'
(Other definitions for north that I've seen before include "PM 1770-82" , "Arctic" , "Boreas was the god of the - wind" , "Direction of Edinburgh from Cardiff" , "Lord -, PM; bearing" .)