Second warning in case of misprint (6)
I believe the answer is:
moment
'second' is the definition.
(I know that second can be written as moment)
'warning in case of misprint' is the wordplay.
'warning' becomes 'omen' (omen can mean a warning or sign of the future).
'in' indicates putting letters inside.
'case of' suggests removing the centre (like emptying a case).
'misprint' with its middle removed is 'mt'.
'omen' put within 'mt' is 'MOMENT'.
(Other definitions for moment that I've seen before include "Second, flash" , "An instant in time - important" , "The present Maureen intended, we hear" , "Brief second" , "An exact point of time" .)