Half Greeks back showing some work (3)
I believe the answer is:
'some work' is the definition.
(an erg is a unit of work done)
'half greeks back' is the wordplay.
'half' suggests halving the letters.
'back' is a reversal indicator.
'greeks' halved is 'gre'.
'gre' backwards is 'ERG'.
'showing' is the link.
(Other definitions for erg that I've seen before include "What's used to record work", "Energy expended", "Unit of work (phys.)", "Shifting dunes in Sahara", "Unit of work; dunes area".)