In tennis, they're especially bad in doubles (6)
I believe the answer is:
faults
'doubles' is the definition.
The answer and definition can be both acts as well as being plural nouns.
Maybe they are linked in a way I don't understand?
I can't explain the rest of the clue.
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" .)