The most common and most likely answer to this crossword clue is the 7 plus 6 letter word KITCHEN MIDDEN
I've seen this crossword clue in The Times
K
I
T
C
H
E
N
M
I
D
D
E
N
Buddy explains!
The straight part
We know that "Old refuse heap" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old refuse heap = kitchen midden.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.