Oldie's line upset for natural preservative (7,3)
I believe the answer is:
linseed oil
'natural preservative' is the definition.
(linseed oil may be used as a preservative)
'oldie's line upset' is the wordplay.
'upset' is an anagram indicator.
'oldies'+'line'='oldiesline'
'oldiesline' with letters rearranged gives 'LINSEED-OIL'.
'for' is the link.
(Other definitions for linseed oil that I've seen before include "what's used in making printer's ink" , "Varnish and paint ingredient" , "Paint/varnish ingredient" , "product out of flax" , "paint additive?" .)