A slow system of exercises? Excellent champion in it making a comeback (3,3)
I believe the answer is:
tai chi
'a slow system of exercises?' is the definition.
(type of exercise)
'excellent champion in it making a comeback' is the wordplay.
'excellent' becomes 'ai' (resembles 'Ac', old-fashioned term for excellent).
'champion' becomes 'ch' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'making a comeback' says the letters should be written backwards.
'it' reversed gives 'ti'.
'ai'+'ch'='aich'
'aich' inserted into 'ti' is 'TAI CHI'.
(Other definitions for tai chi that I've seen before include "Chinese martial art and system of callisthenics" , "Way to cross a fence" , "system of exercises" , "'Chinese form of slow physical exercise (3,3)'" , "form of self-defence" .)