Metric unit of mass (5)
'metric unit of mass' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gram that I've seen before include "Small unit of mass" , "pulses" , "Chickpeas; unit of mass" , "Metric unit of mass" , "Metric unit of weight" .)
(Other definitions for tonne that I've seen before include "Heavyweight" , "1,000 kg" , "Metric weight unit" , "A thousand kilogrammes" , "One thousand kilograms" .)