Shocking extremes of lewdness in Big Brother ? (7)
I believe the answer is:
Here is my best explanation:
'brother ?' is the definition.
'shocking extremes of lewdness in big' is the wordplay.
'shocking' is an anagram indicator (I've seen this in other clues).
'extremes of' means to remove the middle letters.
'lewdness' with its centre removed is 'ls'.
'lsinbig' anagrammed gives 'SIBLING'.
(Other definitions for sibling that I've seen before include "Close relative", "Brother or sister by the same parents", "Maybe brother", "Bro or sis", "A brother or sister", "Bro, perhaps", "Sister or brother" and "One's brother or sister".)