Perhaps 750 grammes some amount where hops dry (4)
I believe the answer is:
kiln
'where hops dry' is the definition.
(hops are dried in a kiln)
'perhaps 750 grammes some amount' is the wordplay.
'perhaps 750 grammes' becomes 'kil' (three quarters of the word 'kilo').
'some amount' becomes 'n' (in maths, n can represent any number).
'kil'+'n'='KILN'
(Other definitions for kiln that I've seen before include "Baker" , "Hops or pottery cooker?" , "Drying furnace" , "Furnace for firing bricks" , "Large oven for baking pottery" .)