Discipline extremely sadistic niece perhaps (7)
I believe the answer is:
science
'discipline' is the definition.
(science is a kind of discipline)
'extremely sadistic niece perhaps' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'perhaps' indicates anagramming the letters.
'sadistic' with its centre taken out is 'sc'.
'niece' with letters rearranged gives 'ience'.
'sc'+'ience'='SCIENCE'
(Other definitions for science that I've seen before include "-- Fiction; -- park" , "Technology" , "Nice sec for branch of knowledge requiring systematic study" , "Branch of study of the physical world" , "Biology, perhaps?" .)