Divine female with one sibling (4)
I believe the answer is:
isis
'divine female' is the definition.
(I've seen this before)
'with one sibling' is the wordplay.
'with' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'sibling' becomes 'sis' (short for sister).
'i'+'sis' is 'ISIS'.
(Other definitions for isis that I've seen before include "The goddess lives twice" , "Revered figure" , "banker in Oxford" , "Jihadists" , "Oxford's second eight" .)