Sibling, base and negative, sent back book (7)
I believe the answer is:
genesis
'book' is the definition.
(first book of the Bible)
'sibling base and negative sent back' is the wordplay.
'sibling' becomes 'sis' (short for sister).
'base' becomes 'e' (mathematical symbol).
'and' means one lot of letters go next to another.
'negative' becomes 'neg' (short for photographic negative).
'sent back' says the letters should be written backwards.
'sis'+'e'+'neg'='siseneg'
'siseneg' back-to-front is 'GENESIS'.
(Other definitions for genesis that I've seen before include "Band; book" , "rock group" , "name of group" , "Bible's first book" , "the start of something" .)