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