The last word in unchanging time and space (9)
I believe the answer is:
firmament
'space' is the definition.
(I've seen this before)
'the last word in unchanging time' is the wordplay.
'the last word' becomes 'amen' (I've seen this before).
'in' is an insertion indicator.
'unchanging' becomes 'firm' (synonyms).
'time' becomes 't' (abbreviation).
'firm'+'t'='firmt'
'amen' going inside 'firmt' is 'FIRMAMENT'.
'and' is the link.
(Other definitions for firmament that I've seen before include "Heavens, sky" , "Sky (literary)" , "outer space, encapsulated" , "The celestial sphere" , "The starry heaven" .)