The most common and most likely answer to this crossword clue is the 9 letter word WASHBOARD
I've seen this crossword clue in The Guardian
W
A
S
H
B
O
A
R
D
Buddy explains!

The straight part
We know that "hard abs" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hard abs = washboard.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 WASHBOARD mean?
WASHBOARD noun- A board with a corrugated surface against which laundry may be rubbed.
- Such a board used as a simple percussion instrument.
- A board fastened along a ship's gunwale to prevent splashing; a splashboard.
- A stretch of ripples or bumps on a dirt or gravel road caused by interaction between traffic and road surface.
- Baseboard; skirting board
- To produce a rippled texture on a surface.
- To play a washboard.
- (bees) To move up and down or back and forth across the surface of a hive, possibly to lay down a layer of propolis and wax.
- A board with a corrugated surface against which laundry may be rubbed.
- Such a board used as a simple percussion instrument.
- A board fastened along a ship's gunwale to prevent splashing; a splashboard.
- A stretch of ripples or bumps on a dirt or gravel road caused by interaction between traffic and road surface.
- Baseboard; skirting board
- To produce a rippled texture on a surface.
- To play a washboard.
- (bees) To move up and down or back and forth across the surface of a hive, possibly to lay down a layer of propolis and wax.