Heap of dust finally placed in bag (5)
I believe the answer is:
stack
'heap' is the definition.
('stack' can be a synonym of 'heap')
'dust finally placed in bag' is the wordplay.
'finally' says to take the final letters.
'placed in' indicates putting letters inside (I've seen this in other clues).
'bag' becomes 'sack' (sack is a kind of bag**).
The final letter of 'dust' is 't'.
't' inserted into 'sack' is 'STACK'.
'of' acts as a link.
(Other definitions for stack that I've seen before include "Place in ordered pile" , "Pile - chimney" , "Aggregation or heap" , "functioned successfully in Scotland" , "Pile of hay" .)