My one is a pit (4)
I believe the answer is:
mine
'my' is the definition.
(I know that mine can be written as my)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for mine that I've seen before include "I made?" , "Store" , "Coalpit" , "Excavate for minerals" , "Pit; belonging to me" .)