Doughnut - foremost refreshment in tourist city - is too sweet (8)
I believe the answer is:
overnice
'doughnut foremost refreshment in tourist city' is the definition.
I can't tell whether this definition defines the answer.
'too sweet' is the wordplay.
'too' becomes 'over' (I can't justify this - if you can you should give a lot more credence to this answer).
'sweet' becomes 'nice' (I've seen this in another clue).
'over'+'nice'='OVERNICE'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for overnice that I've seen before include "Too fastidious" , "too chocolate-boxy" , "Too difficult" , "Excessively fussy" .)