Extend part of journey round North, in that case (8)
I believe the answer is:
lengthen
'extend' is the definition.
(I've seen this before)
'part of journey round north in that case' is the wordplay.
'part of journey' becomes 'leg' ('leg' is part of 'journey').
'round' means one lot of letters goes inside another.
'north' becomes 'n' ('N' can be a synonym of 'north').
'in that case' becomes 'then' ('then' can be a synonym of 'in that case').
'leg' placed around 'n' is 'leng'.
'leng'+'then'='LENGTHEN'
(Other definitions for lengthen that I've seen before include "Make or become longer" , "increase in duration" , "Extend or draw out" , "Stretch" , "Prolong" .)