Lectured on small radii (6)
I believe the answer is:
spokes
'radii' is the definition.
('spoke' can be a synonym of 'radius')
'lectured on small' is the wordplay.
'lectured' becomes 'spoke' (I've seen this in another clue).
'on' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size).
'spoke'+'s'='SPOKES'
(Other definitions for spokes that I've seen before include "Parts of a wheel" , "Radials of a wheel" , "Radial supports on a wheel" , "Bicycle wheel struts" .)