About to break two arms badly, making a heap of soil (4,4)
I believe the answer is:
worm cast
'a heap of soil' is the definition.
(heap of soil formed by a worm)
'about to break two arms badly' is the wordplay.
I cannot really see how this works, but
'about' could be 'ca' (short for circa) and 'ca' is located in the answer.
This explanation may well be incorrect...
'making' is the link.
(Other definitions for worm cast that I've seen before include "a ground-breaking result, in a small way" , "beach feature" , "bit of dirt that's been dug up" , "Small coil of earth or sand" .)