Alterations are made when the last pieces are viewed (4)
I believe the answer is:
seen
'are viewed' is the definition.
(to view is to see)
'alterations are made when the last' is the wordplay.
'the last' says to take the final letters.
The last letters of 'alterations are made when' are 'SEEN'.
'pieces' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for seen that I've seen before include "Noticed" , "Sighted" , "Perceived by sight" , "Spotted" , "Distinguished" .)