Changes the last word, originally deemed substandard (6)
I believe the answer is:
amends
'changes' is the definition.
(amending is a kind of changing)
'the last word originally deemed substandard' is the wordplay.
'the last word' becomes 'amen' (I've seen this before).
'originally' suggests taking the first letters.
The first letters of 'deemed substandard' is 'ds'.
'amen'+'ds'='AMENDS'
(Other definitions for amends that I've seen before include "Alters and improves" , "Frees from error" , "Make these to make up for past wrongs" , "Alters, makes right" , "It is made in reparation" .)