Hydrocarbon, brown, stored in old church (6)
I believe the answer is:
octane
'hydrocarbon' is the definition.
(octane is a kind of hydrocarbon)
'brown stored in old church' is the wordplay.
'brown' becomes 'tan' (tan is a kind of brown).
'stored in' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'church' becomes 'ce' (abbreviation for Church of England).
'o'+'ce'='oce'
'tan' going within 'oce' is 'OCTANE'.
(Other definitions for octane that I've seen before include "Anti-knock rating" , "Hydrocarbon found in petrol - at once (anag)" , "Hydrocarbon present in petrol" , "chemical" , "Flammable hydrocarbon - at once (anag)" .)