Game and ultimately Margaret Thatcher maybe losing lead (6)

I believe the answer is:

tennis

I'm a little stuck... Click here to teach me more about this clue!

'game' is the definition.
(tennis is a game)

I don't understand the remainder of the clue.

(Other definitions for tennis that I've seen before include "Sent in, strangely, for sport" , "Net sport" , "Racket-and-ball game" , "Real or lawn game" , "Sport Dwight Davis gave trophy for" .)

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.