Spring containing unrefined petroleum (5,3)
I believe the answer is:
crude oil
'petroleum' is the definition.
(synonyms)
'spring containing unrefined' is the wordplay.
'spring' becomes 'coil' (coil spring is a kind of spring).
'containing' means one lot of letters goes inside another.
'unrefined' becomes 'rude' (similar in meaning).
'coil' placed around 'rude' is 'CRUDE OIL'.
(Other definitions for crude oil that I've seen before include "liquid asset" , "From which petrol is made" , "output from rig" , "Source of fuel" , "Unrefined petroleum" .)