Definite final word: leader in television is Sky (9)
I believe the answer is:
firmament
'is sky' is the definition.
(the firmament means the sky)
'definite final word leader in television' is the wordplay.
'definite' becomes 'firm' (firm can mean definite or decided).
'final word' becomes 'amen' (last word in a prayer).
'leader in' suggests taking the first letters.
The first letter of 'television' is 't'.
'firm'+'amen'+'t'='FIRMAMENT'
(Other definitions for firmament that I've seen before include "Heavens, sky" , "The celestial sphere" , "The starry heaven" , "Sky (literary)" , "outer space, encapsulated" .)