The most common and most likely answer to this crossword clue is the 9 letter word PONDEROUS
I've seen this crossword clue in The Daily Telegraph
P
O
N
D
E
R
O
U
S
Buddy explains!

The straight part
We know that "heavy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case heavy = ponderous.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.
What does PONDEROUS mean?
PONDEROUS adjective- Heavy, massive, weighty.
- (by extension) Serious, onerous, oppressive.
- Clumsy, unwieldy, or slow, especially due to weight.
- Dull, boring, tedious; long-winded in expression.
- Characterized by or associated with pondering.
- Dense.