Stack (4)
I think it could be either:
'stack' is the definition.
(I know that stack can be written as pile)
This is the entire clue.
(Other definitions for pile that I've seen before include "lot of money" , "large amount of money" , "large building" , "Part of reactor" , "Fortune" .)
'stack' is the definition.
(I know that stack can be written as heap)
This is all the clue.
(Other definitions for heap that I've seen before include "Unreliable old car" , "Rough pile" , "pile-up" , "Mound - dilapidated vehicle" , "Mound, stack" .)