It's allowed in tennis (3)
I believe the answer is:
let
This could be a double definition.
'it's allowed' is the first definition.
(to let is to allow or permit)
'tennis' is the second definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'in' is the link.
Can you help me to learn more?
(Other definitions for let that I've seen before include "Allow; permit" , "Allow; rental period" , "Allow - hire" , "Rent out (property)" , "Allow the obstruction" .)