It's one's duty to cross this out (6)
I believe the answer is:
excise
'out' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'it's one's duty to cross' is the wordplay.
I cannot quite see how this works, but
'one's' could be 'is' (Roman numeral) and 'is' is present in the answer.
'cross' could be 'x' (an X is a cross) and 'x' is found in the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is located in the answer.
This explanation may well be incorrect...
'this' acts as a link.
Can you help me to learn more?
(Other definitions for excise that I've seen before include "Duty; cut out" , "Tax on certain goods and commodities" , "Internal tax" , "Remove by cutting out" , "Type of tax" .)