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' is an insertion indicator.
'unrefined' becomes 'rude' (similar in meaning).
'coil' going 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" .)