One day this person will be caught out (5,4)
I believe the answer is:
april fool
'one day this person will be caught out' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'person will be caught out' is the wordplay.
I cannot really understand how this works, but
'person' could be 'fool' (fool is a kind of person) and 'fool' is found within the answer.
The remaining letters 'april' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for april fool that I've seen before include "seasonal jape" , "Spring hoax victim" , "12 [POISSON DAVRIL]" , "One taken in" , "Seasonal victim" .)