Of the upper hipbone (5)
I believe the answer is:
'of the upper hipbone' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for iliac that I've seen before include "Pertaining to hip bone" , "bottom of a hip bone" , "Like certain body parts" , "Relating to part of the hip bone" , "hipbone" .)