Encoding device way above top of mountain (5)
I believe the answer is:
modem
'encoding device' is the definition.
'modem' can be an answer for 'device' (I have seen 'electronic device' mean 'modem' so perhaps 'device' could also mean 'modem'). I'm unsure of the 'encoding' bit.
'way above top of mountain' is the wordplay.
'way' becomes 'mode' (synonyms).
'above' says to put letters next to each other.
'top of' suggests taking the first letters.
The initial letter of 'mountain' is 'm'.
'mode'+'m'='MODEM'
Can you help me to learn more?
(Other definitions for modem that I've seen before include "Device for connecting two computers" , "Phone line and computer connector" , "Device to connect a computer to a telephone line" , "Computer interface" , "Telephonic computer connection" .)