A measure of underground work? (3)
I believe the answer is:
erg
'a measure' is the definition.
(erg is a kind of measure)
'of underground work?' is the wordplay.
'of' says the answer is hidden in the clue.
'ERG' is hidden within 'underground work'.
(Other definitions for erg that I've seen before include "shifting sand" , "What's used to record work" , "Unit of work or energy" , "Shifting dunes in Sahara" , "Unit of work; dunes area" .)