Cry that is about to reverberate endlessly (7)
I believe the answer is:
screech
'cry' is the definition.
(screech is a kind of cry)
'that is about to reverberate endlessly' is the wordplay.
'that is' becomes 'sc' (abbrv. for Latin scilicet; 'namely').
'about' becomes 're' (regarding**).
'to reverberate' becomes 'echo' ('echo' can be a synonym of 'reverberate').
'endlessly' means to remove the last letter.
'echo' with its final letter removed is 'ech'.
'sc'+'re'+'ech'='SCREECH'
(Other definitions for screech that I've seen before include "Harsh piercing cry" , "Outcry" , "Squawk" , "Terrifying sound" , "Squeal" .)