The most common and most likely answer to this crossword clue is the 8 letter word MULTIGYM
I've seen this crossword clue in The Times
M
U
L
T
I
G
Y
M
Buddy explains!

The straight part
We know that "One good in exercising large tummy?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one good in exercising large tummy? = multigym.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 MULTIGYM mean?
MULTIGYM noun- A gym consisting of several items of weightlifting apparatus.