In decimal it represents one cubic decimetre (5)
I believe the answer is:
litre
'one cubic decimetre' is the definition.
('litre' can be a synonym of 'cubic decimetre')
'in decimal it represents' is the wordplay.
'in' indicates the answer is hidden within the clue.
'LITRE' is hidden in the letters of 'decimal it represents'.
(Other definitions for litre that I've seen before include "Fluid measure" , "Unit of liquid measure" , "Measure of liquid volume" , "Metric measure of capacity" , "Nearly two pints" .)