Light unit measurement's European norm initially (5)
I believe the answer is:
lumen
'light' is the definition.
(thesaurus)
'unit measurement's european norm initially' is the wordplay.
'unit' becomes 'lu' (LU is an example**).
'initially' says to take the initial letters.
The initial letters of 'measurements european norm' is 'men'.
'lu'+'men'='LUMEN'
(Other definitions for lumen that I've seen before include "Unit of light flux" , "Unit of luminous flux" .)