Immature adult — a Parisian lacking discipline (4)
I believe the answer is:
yoga
'discipline' is the definition.
(I have seen 'Ascetic discipline ' mean 'yoga' so perhaps 'discipline' could also mean 'yoga')
'immature adult a parisian lacking' is the wordplay.
'immature' becomes 'young' ('young' can be a synonym of 'immature').
'adult' becomes 'a'.
'a parisian' becomes 'un'.
'lacking' is a deletion indicator.
'young'+'a'='younga'
'younga' with 'un' taken away is 'YOGA'.
(Other definitions for yoga that I've seen before include "path to enlightenment" , "Healthy exercises" , "exercise regime" , "Hindu spiritual and ascetic discipline" , "Hindu system of meditation and self-control" .)