Force of lecture on the radio (6)
I believe the answer is:
torque
'force' is the definition.
(force in physics)
'lecture on the radio' is the wordplay.
'lecture' becomes 'talk' (synonyms).
'on the radio' shows a homophone (sound like).
'talk' sounds like 'TORQUE'.
'of' is the link.
(Other definitions for torque that I've seen before include "Twisting force; metal necklace" , "Rotating force" , "Rotating forces" , "Metal necklace; also a measure of force" , "Quoter (anag.)" .)