Team sports master's doubled upper limits (6)
I believe the answer is:
maxima
'upper limits' is the definition.
('maximum' can be a synonym of 'upper limit')
'team sports master's doubled' is the wordplay.
'team' becomes 'XI' (a football team, from Roman numerals for 11).
'sports' means one lot of letters goes inside another (inserted letters wear or 'sport' others).
'master' becomes 'ma' (Master of Arts).
'doubled' says to double the letters.
'ma' duplicated is 'mama'.
'xi' put within 'mama' is 'MAXIMA'.
(Other definitions for maxima that I've seen before include "greatest possible amounts" , "Highest points" , "High points" , "Largest amounts" , "peaks" .)