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