Something identifying sibling, primarily (3)
I believe the answer is:
sis
'something identifying sibling primarily' is the definition.
I can't tell whether this defines the answer.
'something identifying sibling primarily' is the wordplay.
'primarily' suggests taking the first letters.
The initial letters of 'something identifying sibling' is 'SIS'.
(Other definitions for sis that I've seen before include "spies ..." , "Little sibling" , "relative" , "Relative's" , "Secret Intelligence Service (abbr.)" .)