Unit of work therefore incomplete (3)
I believe the answer is:
erg
'unit of work' is the definition.
(I've seen this before)
'therefore incomplete' is the wordplay.
'therefore' becomes 'ergo' (I've seen this before).
'incomplete' means to remove the last letter.
'ergo' with its last letter removed is 'ERG'.
(Other definitions for erg that I've seen before include "Unit of work or energy" , "Unit of work (phys.)" , "Unit of work; dunes area" , "Energy expended" , "Shifting dunes in Sahara" .)