Cancel because having entered before (5)
I believe the answer is:
erase
'cancel' is the definition.
(I know that erase is a type of cancel)
'because having entered before' is the wordplay.
I cannot quite see how this works, but
'because' could be 'as' (I've seen this in other clues) and 'as' is found within the answer.
'before' could be 'e' (I've seen this in other clues) and 'e' is found in the answer.
The remaining letters 're' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for erase that I've seen before include "Remove evidence of" , "Kill" , "Scratch" , "Blank out" , "Do away with" .)