Start of operation with nothing on time (4,4)
I believe the answer is:
zero hour
'start of operation' is the definition.
(I've seen this before)
'nothing on time' is the wordplay.
'nothing' becomes 'zero' (synonyms).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'time' becomes 'hour' (unit of time).
'zero'+'hour'='ZERO HOUR'
'with' acts as a link.
(Other definitions for zero hour that I've seen before include "Kick-off" , "Time action begins" , "Exact time of starting operation" , "When battle is due to commence" , "when operation is due to start" .)