Mean to alter 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' is an anagram of 'AMEN'.
(Other definitions for amen that I've seen before include "Word at the end of a prayer" , "Name for end to prayer" , "Name for prayer's end" , "Approve" , "Prayerful last word" .)