Part of SRO
I believe the answer is:
only
'part of sro' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for only that I've seen before include "Existing alone with no other, perhaps a child" , "Siblingless (child)" , "Simply" , "Alone; solitary" , "Except that; single" .)