Tennis champion, one for a cocktail (7)
I believe the answer is:
martini
'for a cocktail' is the definition.
(martini is a kind of cocktail)
'tennis champion one' is the wordplay.
'tennis champion' becomes 'martin' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'one' becomes 'i' (Roman numeral).
'martin'+'i'='MARTINI'
Can you help me to learn more?
(Other definitions for martini that I've seen before include "James Bond's drink of choice" , "tipple" , "Cocktail of gin and dry vermouth" , "Alternative name for Simone Memmi, pre-Renaissance Italian painter" , "In glass, one's dry" .)