About a second before, cancel (5)
I believe the answer is:
erase
'cancel' is the definition.
(I know that erase is a more specific form of the action cancel)
'about a second before' is the wordplay.
'about' is an insertion indicator.
'second' becomes 's' (common abbreviation).
'before' becomes 'ere' (poetic term for 'before').
'a'+'s'='as'
'as' going within 'ere' is 'ERASE'.
(Other definitions for erase that I've seen before include "Rub out, cancel" , "To remove all trace" , "Remove evidence of" , "Eliminate" , "Obliterate" .)