Being just partly found out can be one's downfall! (4)
I believe the answer is:
undo
'being just partly found out can be one's downfall' is the definition.
I can't judge whether this defines the answer.
'one's downfall' is the wordplay.
'one' becomes 'un' (I've seen this in other clues).
'downfall' becomes 'do' (I can't justify this - if you can you should give a lot more credence to this answer).
'un'+'do'='UNDO'
Can you help me to learn more?
(Other definitions for undo that I've seen before include "Bring to ruin" , "Subvert" , "set free" , "Untie or ruin" , "Unfasten, loosen" .)