Still to go North during end of day, not South (6)
I believe the answer is:
unsent
'still to go' is the definition.
(still to be sent)
'north during end of day not south' is the wordplay.
'north' becomes 'N' (synonyms).
'during' means one lot of letters goes inside another.
'end of day' becomes 'sunset' (the sunset happens at the end of the day).
'not' indicates named letters should be taken away.
'south' becomes 'S' (synonyms).
'n' put into 'sunset' is 'sunsent'.
'sunsent' with 's' taken away is 'UNSENT'.
(Other definitions for unsent that I've seen before include "not transmitted" , "Not dispatched" .)