Change last word of prayer, improved ultimately (5)
I believe the answer is:
amend
'change' is the definition.
(amending is a kind of changing)
'last word of prayer improved ultimately' is the wordplay.
'last word of prayer' becomes 'amen' (I've seen this before).
'ultimately' says to take the final letters.
The final letter of 'improved' is 'd'.
'amen'+'d'='AMEND'
(Other definitions for amend that I've seen before include "Modify, correct" , "Put right" , "make an improvement" , "Revise" , "Make small improvements to" .)