Standard Oil's tar sands trip established reserves (5,3,7)
I believe the answer is:
stars and stripes
'standard' is the definition.
(the US flag - standard can mean a flag)
'tar sands trip established reserves' is the wordplay.
'trip' indicates an anagram.
'established reserves' becomes 'stripes' (I can't justify this - if you can you should believe this answer much more).
'tar'+'sands'='tarsands'
'tarsands' is an anagram of 'starsand'.
'starsand'+'stripes'='STARS AND STRIPES'
'oil's' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for stars and stripes that I've seen before include "indicators of military rank" , "indications of rank" , "American flier" , "The US flag" , "Standard" .)