Bets they're stand-ins (7)
I believe the answer is:
doubles
This clue might be a double definition.
'bets' is the first 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?
'stand-ins' is the second definition.
The answer and definition can be both people as well as being plural nouns.
Perhaps there's an association between them I don't understand?
'they're' is the link.
Can you help me to learn more?
(Other definitions for doubles that I've seen before include "Like Carton and Darnay" , "sort of game" , "Made twice as much" , "Tennis match for four players" , "match in court" .)