Preserve the end of past prestige (6)
I believe the answer is:
cachet
'prestige' is the definition.
('cachet' can be a synonym of 'prestige')
'preserve the end of past' is the wordplay.
'preserve' becomes 'cache'.
'the end of' suggests the final letters.
The last letter of 'past' is 't'.
'cache'+'t'='CACHET'
(Other definitions for cachet that I've seen before include "Distinguishing mark or seal" , "Official seal on document" , "French prestige or distinguishing mark" , "Seal; mark of prestige" , "Distinguishing feature" .)