Short girl starts to revel in race for water treatment (12)
I believe the answer is:
chlorination
'water treatment' is the definition.
(treating water with chlorine)
'short girl starts to revel in race' is the wordplay.
'short girl' becomes 'chlo' (I am not sure about the 'short' bit.).
'starts to' indicates taking the first letters.
'race' becomes 'nation' (both can mean a national or ethnic group).
The first letters of 'revel in' is 'ri'.
'chlo'+'ri'+'nation'='CHLORINATION'
'for' is the link.
(Another definition for chlorination that I've seen is " water treatment".)