Obscure extra had to interrupt broadcast (10)
I believe the answer is:
overshadow
'obscure' is the definition.
(overshadowing is a kind of obscuring)
'extra had to interrupt broadcast' is the wordplay.
'extra' becomes 'over'.
'to interrupt' means one lot of letters goes inside another.
'broadcast' becomes 'sow' (sowing is a kind of broadcasting**).
'had' placed into 'sow' is 'shadow'.
'over'+'shadow'='OVERSHADOW'
(Other definitions for overshadow that I've seen before include "Cast gloom on" , "Dominate" , "Dwarf" , "Eclipse" .)