Mean to alter the last word (4)
I believe the answer is:
amen
'last word' is the definition.
(last word in a prayer)
'mean to alter' is the wordplay.
'to alter' is an anagram indicator.
'mean' with letters rearranged gives 'AMEN'.
'the' acts as a link.
(Other definitions for amen that I've seen before include "Conclusion of prayer" , "Word of approval" , "Extremist" , "Name for end to prayer" , "Final word in prayer" .)