It's allowed in tennis (3)
I believe the answer is:
let
This clue might be a double definition.
'it's allowed' is the first definition.
(to let is to allow or permit)
'tennis' is the second definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe there's a link between them 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" .)