Broadcast without finishing one's lines (5)
I believe the answer is:
radii
'lines' is the definition.
(radius is a kind of line)
'broadcast without finishing one's' is the wordplay.
'broadcast' becomes 'radio' (radio broadcast is a kind of broadcast).
'without finishing' means to remove the last letter.
'one' becomes 'i' (Roman numeral).
'radio' with its final letter removed is 'radi'.
'radi'+'i'='RADII'
(Other definitions for radii that I've seen before include "Spokes of wheel - bones of arm" , "spoke with others" , "Veins of insect's wings" , "Spokes of a circle" , "Bones of the human forearm" .)