Next to a cable, a couple of letters can seem heavy (6)
I believe the answer is:
leaden
'heavy' is the definition.
(similar in meaning)
I don't understand the rest of the clue.
Can you help me to learn more?
(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" .)