Nothing seen in senility (3)
I believe the answer is:
nil
'nothing' is the definition.
(eg nil is zero in football)
'in senility' is the wordplay.
'in' says the answer is hidden in the clue.
'NIL' is hidden within 'senility'.
'seen' acts as a link.
(Other definitions for nil that I've seen before include "Lacking value" , "none" , "Nothing, zero" , "Nothing, zilch" , "Nothing, naught" .)