Mean to change last word (4)
I believe the answer is:
amen
'last word' is the definition.
(last word in a prayer)
'mean to change' is the wordplay.
'to change' indicates anagramming the letters.
'mean' with letters rearranged gives 'AMEN'.
(Other definitions for amen that I've seen before include "I agree!" , "Word of approval" , "Extremist" , "Name for prayer's end" , "Churchgoer's assent?" .)