Improved security devices from Spooner where the envelope is pushed? (9)

18D — Improved security devices from Spooner where the envelope is pushed? (9) — LETTERBOX

• Definition
  Definition found: 'envelope is'
• Indicator
  Indicator found: 'Spooner'
• Operation
  'Spooner' signals a homophone operation
• Fodder
  Fodder: 'Improved security devices'
• Synonym
  Synonyms spotted: 'Improved' → BETTER
• Synonym
  Synonyms spotted: 'security devices' → LOCKS
• Assembly
  BETTER + LOCKS > BETTERLOCKS = LETTERBOX

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