High Street store contracted Northern Gas (5)
I believe the answer is:
argon
'gas' is the definition.
(argon is a gas)
'high street store contracted northern' is the wordplay.
'high street store' becomes 'Argos' (UK high street retailer).
'contracted' means to remove the last letter (to contract is to shorten).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'argos' with its last letter taken away is 'argo'.
'argo'+'n'='ARGON'
(Other definitions for argon that I've seen before include "rare gas" , "Chemical element" , "Gas used in electric lights" , "Noble gas used in light bulbs" , "Gas, inert constituent of air" .)