Where one first saw the light (10)
I believe the answer is:
birthplace
'where one' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'first saw the light' is the wordplay.
'first' becomes 'birth' (birth is a kind of first).
'saw the light' becomes 'place' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'birth'+'place'='BIRTHPLACE'
Can you help me to learn more?
(Other definitions for birthplace that I've seen before include "Where one starts life" , "Place where something began" , "Where everyone began" , "Shakespeare's was Stratford" , "Queen, London, 1926" .)