Heaven beginning to tremble after decisive end of prayer (9)
I believe the answer is:
firmament
'heaven' is the definition.
(thesaurus)
'beginning to tremble after decisive end of prayer' is the wordplay.
'beginning to' suggests taking the first letters.
'after' is a charade indicator (letters next to each other).
'decisive' becomes 'firm'.
'end of prayer' becomes 'amen' (I've seen this before).
The first letter of 'tremble' is 't'.
'firm'+'amen'='firmamen'
't' put after 'firmamen' is 'FIRMAMENT'.
(Other definitions for firmament that I've seen before include "Heavens, sky" , "outer space, encapsulated" , "The celestial sphere" , "Sky (literary)" , "The starry heaven" .)