Nothing can be added to day in Scottish town (6)
I believe the answer is:
thurso
'scottish town' is the definition.
I know nothing about this answer so I can't tell whether this works.
'nothing can be added to day' is the wordplay.
I cannot really understand how this works, but
'nothing' could be 'o' (O resembles 0) and 'o' is present in the answer.
'day' could be 'thurs' (short for Thursday) and 'thurs' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'in' is the link.
Can you help me to learn more?
(Other definitions for thurso that I've seen before include "Northernmost town on the UK mainland" , "Northernmost town of mainland Britain" , "Town near Wick in Scotland" , "Northern Scottish town" , "Town of N. Scotland" .)