Some work is therefore cut short (3)
I believe the answer is:
Here is my best explanation:
'some work' is the definition.
(an erg is a unit of work done)
'therefore cut short' is the subsidiary indication.
'therefore' becomes 'ergo'.
'cut short' means to remove the last letter.
'ergo' with its final letter taken off is 'erg'.
'is' is the link.
(Other definitions for erg that I've seen before include "Unit of work; dunes area", "shifting sand", "Work unit", "Amount of work", "A unit of work", "amount", "Unit of work (phys.)", "Energy expended", "Shifting dunes in Sahara" and "Unit of measurement of work".)