See nothing in senility (3)
I believe the answer is:
nil
'see nothing' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'in senility' is the wordplay.
'in' indicates the answer is hidden within the clue.
'NIL' is hidden in the letters of 'senility'.
Can you help me to learn more?
(Other definitions for nil that I've seen before include "Non-existent" , "Nothing, no value" , "Zero, zilch" , "Nothing found in senility" , "Latin zero" .)