Next to a cable, a couple of letters can seem heavy (6)

I believe the answer is:

leaden

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'heavy' is the definition.
(similar in meaning)

I cannot understand how the rest of the clue works.

(Other definitions for leaden that I've seen before include "Slow-moving, being like heavy metal" , "Gloomy - slow-moving" , "Heavy - overcast" , "Oppressive" , "Leaned over in slow, heavy way" .)

I've seen this clue in the Evening Standard.

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