### Clue: Mac adding strength of spirit to general excellence (10)
### Answer: WATERPROOF 
 ### Explanation: Definition: Mac
 PROOF(a scaled measure of the strength of distilled alcoholic spirits) **plus**(adding … to) WATER(excellence/quality, as in the degree of brilliance in a diamond, and in the phrase "of the first water" – I presume "general" is included for the surface reading).
Defn: A garment that keeps out water, an example of which is the mac, short for "mackintosh".