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' indicates a hidden word.
'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" .)