Mound is a new work: last of earth going in (7)
I believe the answer is:
anthill
'mound' is the definition.
(anthill is a kind of mound)
'is a new work last of earth going in' is the wordplay.
'is a new' becomes 'an' (I've seen this in other clues).
'work' becomes 'till' (tilling is a kind of working**).
'last of' says to take the final letters.
'going in' means one lot of letters goes inside another.
The last letter of 'earth' is 'h'.
'till' enclosing 'h' is 'thill'.
'an'+'thill'='ANTHILL'
(Other definitions for anthill that I've seen before include "Home for social workers?" , "Mound of insects" , "Many a worker here" , "Nest" , "lots of workers" .)