Cancel deleterious letters (6)
I believe the answer is:
delete
'cancel deleterious letters' is the definition.
'delete' can be an answer for 'cancel' (synonyms). I am not sure about the 'deleterious letters' bit.
'cancel deleterious letters' is the wordplay.
'cancel' becomes 'dele' (I've seen this in another clue).
'deleterious letters' becomes 'te' (I am not sure about this - if you are sure you should believe this answer much more).
'dele'+'te'='DELETE'
Can you help me to learn more?
(Other definitions for delete that I've seen before include "Wipe out" , "Erase or destroy" , "Rub out, expunge" , "Strikeout, remove" , "'Erase, remove (6)'" .)