Tennis's Becker (5)
I believe the answer is:
boris
'tennis's becker' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for boris that I've seen before include "- Johnson, Mayor of London" , "- Godunov, Russian tsar" , "Russian name" , "- Godunov, Mussorgsky opera" , "Leading Brexiteer's given name" .)