### Clue: One ounce at first may be bracing (5)
### Answer: OZONE 
 ### Explanation: Definition: may be bracing
  OZ(abbrev. for ounce weight) **placed before**(at first) ONE. 
Answer: The British Victorian folk myth was that the characteristic smell of the sea, carried by the breeze, was "bracing" and that the smell was due to ozone.  Wrong on both counts – the smell is due to dimethyl suphide, a compound formed by phytoplankton, and which is toxic in high concentrations.  As is ozone, nasty stuff but very useful in preventing ultraviolet rays from space from reaching plants and animals.