Understood everything, when one watched right through (3,2,3)
I believe the answer is:
saw it all
'watched right through' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'understood everything when one' is the wordplay.
I cannot quite understand how this works, but
'understood' could be 'saw' ('see' can be a synonym of 'understand') and 'saw' is located in the answer.
'everything' could be 'all' (all things) and 'all' is found within the answer.
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
A single letter 't' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?