Cancel Caesarean section on reflection (5)
I believe the answer is:
erase
'cancel' is the definition.
(I know that erase is a more specific form of the action cancel)
'caesarean section on reflection' is the wordplay.
'section' says the answer is hidden in the clue.
'on reflection' shows that the letters should be reversed in order.
'esare' is hidden within 'caesarean'.
'esare' written backwards gives 'ERASE'.
(Other definitions for erase that I've seen before include "score" , "Clear" , "Delete" , "Put out of mind" , "Blot out" .)