Slash more antiquated energy store (9)

I believe the answer is:

gasholder

'energy store' is the definition.

'slash more antiquated' is the wordplay.
'slash' becomes 'gash' ('gash' can be a synonym of 'slash').
'more antiquated' becomes 'older' ('old' can be similar in meaning to 'antiquated').
'gash'+'older'='GASHOLDER'

(Other definitions for gasholder that I've seen before include "Heard logs (anag) - fuel store" , "Storage tank" , "fuel storage facility" , "Fuel tank" .)

I've seen this clue in The Times.

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