Echo on an empty terminal is endless (7)
I believe the answer is:
eternal
'endless' is the definition.
(I've seen this in another clue)
'echo on an empty terminal' is the wordplay.
'echo' becomes 'e' (phonetic alphabet: alpha, bravo, charlie etc.).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'an empty' says to hollow out the word (remove centre letters).
'terminal' with its centre taken out is 'ternal'.
'e'+'ternal'='ETERNAL'
'is' is the link.
(Other definitions for eternal that I've seen before include "Immortal" , "carrying on endlessly" , "going on for ever" , "Imperishable" , "Everlasting, not affected by time" .)