Untidy collection in the apartment (4)
I believe the answer is:
heap
'untidy collection' is the definition.
(a heap is an untidy set of things)
'in the apartment' is the wordplay.
'in' indicates a hidden word.
'HEAP' is hidden in the letters of 'the apartment'.
(Other definitions for heap that I've seen before include "Loose pile" , "Mound; old car" , "Mound, pile" , "Mound - dilapidated vehicle" , "Mound, stack" .)