Alter the last word at end of screed (5)
I believe the answer is:
amend
'alter' is the definition.
(amending is a kind of altering)
'the last word at end of screed' is the wordplay.
'the last word' becomes 'amen' (last word in a prayer).
'at end of' suggests the final letters.
The last letter of 'screed' is 'd'.
'amen'+'d'='AMEND'
(Other definitions for amend that I've seen before include "Put right" , "Change to improve" , "Correct or alter to improve" , "Make minor improvements to (something)" , "Make minor improvements to (a document)" .)