Direct lines to centre one added to incomplete set? (5)
I believe the answer is:
Here is my best explanation:
'direct lines to centre' is the definition.
'one added to incomplete set?' is the subsidiary indication.
'one added' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'to' says to put letters next to each other (I've seen this in other clues).
'incomplete' means to remove the last letter.
'set?' becomes 'radio' (radio is a kind of set).
'radio' with its last letter taken away is 'radi'.
'i' after 'radi' is 'radii'.
(Other definitions for radii that I've seen before include "Lines from a circle's centre to the circumference", "Lines between the center and circumference of a circle", "They meet in the middle of a circle", "Spokes of wheel - bones of arm", "Halves of diameters", "Halves of diameter", "Half-diameters", "Two making up diameter", "Bones of the human forearm" and "More than one spoke".)