Memorial King George originally built for unit (10)
I believe the answer is:
kilogramme
'unit' is the definition.
(I know that kilogramme is a coherent SI unit)
'memorial king george originally built' is the wordplay.
'originally' says to take the initial letters.
'built' indicates anagramming the letters (the same letters can be used to build the answer).
The initial letters of 'king george' is 'kg'.
'memorial'+'kg'='memorialkg'
'memorialkg' anagrammed gives 'KILOGRAMME'.
'for' is the link.
(Other definitions for kilogramme that I've seen before include "Metric weight" , "SI unit" .)