The most common and most likely answer to this crossword clue is the 4 plus 4 plus 7 letter word LINE ONES POCKETS
I've seen this crossword clue in Financial Times
L
I
N
E
O
N
E
S
P
O
C
K
E
T
S
Buddy explains!
The straight part
We know that "Make a heap of money" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case make a heap of money = line ones pockets.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.