First signs of rickets, as displayed in immature bones (5)
I believe the answer is:
radii
'bones' is the definition.
(radius is a kind of bone)
'first signs of rickets as displayed in immature' is the wordplay.
'first signs of' says to take the initial letters.
The first letters of 'rickets as displayed in immature' is 'RADII'.
(Other definitions for radii that I've seen before include "Spokes of wheel - bones of arm" , "Straight lines from centre of a circle" , "Geometrical lines" , "parts for arms" , "Veins of insect's wings" .)