Looks to me like a severely etiolated Cereus peruvianus ; cut it down by half below the weakened 'stretchy' bit in the middle . It shouldn't be that shape .
Place in g/house or sunny conservatory ; let the top callous over and then water as normal . It's getting a bit late in the year now for much more growth ; next year should see branches develop , but remember ; give MAXIMUM sunshine for strong growth .
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