The most common and most likely answer to this crossword clue is the 3 plus 3 plus 2 plus 3 plus 4 letter word THE FAT OF THE LAND
I've seen this crossword clue in The Daily Telegraph
T
H
E
F
A
T
O
F
T
H
E
L
A
N
D
Buddy explains!
The straight part
We know that "Is this what produces the nation’s heavyweights" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case is this what produces the nation’s heavyweights = the fat of the land.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.