Nothing sent out in beginning (5)
I believe the answer is:
onset
'beginning' is the definition.
(onset is a kind of beginning)
'nothing sent out in' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'out' is an anagram indicator (out can mean wrong or inaccurate).
'in' means one lot of letters goes inside another.
'sent' anagrammed gives 'nset'.
'o' going into 'nset' is 'ONSET'.
(Other definitions for onset that I've seen before include "Advent" , "Assault; start" , "Appearance of" , "First signs" , "starting?" .)