Crossword Genius

In tennis, they’re especially bad in doubles (6)

Ross

I believe the answer is:

faults

I'm a little stuck... Click here to teach me more about this clue!

'doubles' is the definition.
The definition and answer can be both acts as well as being plural nouns.
Maybe you can see an association between them that I don't see?

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for faults that I've seen before include "Weakness or failings" , "Flaws or failings" , "Defects and failings" , "'Flaws, imperfections (6)'" , "Flaws - defects" .)

I've seen this clue in the Evening Standard.
Want a hint initially instead of a full solution? Install my app