Repeat every one without a zero (4)
I believe the answer is:
echo
'repeat' is the definition.
(I know that repeat can be written as echo)
'every one without a zero' is the wordplay.
'every one' becomes 'each' (I've seen this in other clues).
'without' suggests deleting specific letters.
'zero' becomes 'O' (O resembles 0).
'each' with 'a' taken out is 'ech'.
'ech'+'o'='ECHO'
(Other definitions for echo that I've seen before include "one catches on the rebound?" , "repercussion" , "next to delta" , "Send back" , "Reflected sound" .)