Some work thus finishes early (3)
I believe the answer is:
erg
'some work' is the definition.
(an erg is a unit of work done)
'thus finishes early' is the wordplay.
'thus' becomes 'ergo' (I've seen this in another clue).
'finishes early' means to remove the last letter (the word finishes before its final letter).
'ergo' with its last letter taken away is 'ERG'.
(Other definitions for erg that I've seen before include "shifting sand" , "Energy expended" , "Unit of work or energy" , "Shifting dunes in Sahara" , "Unit of measurement of work" .)