Midday? No, almost one (4)
I believe the answer is:
noon
'midday?' is the definition.
(I know that midday can be written as noon)
'no almost one' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'one' with its final letter taken away is 'on'.
'no'+'on'='NOON'
(Other definitions for noon that I've seen before include "[LUNCHTIME] ?" , "Twelve o'clock in the day" , "when PM starts" , "12 o'clock" , "12 pm" .)