Notice nothing in being decrepit (6)
I believe the answer is:
senile
'decrepit' is the definition.
(thesaurus)
'notice nothing in' is the wordplay.
'notice' becomes 'see' (I've seen this before).
'nothing' becomes 'nil' ('nil' can be a synonym of 'nothing').
'in' indicates putting letters inside.
'see' placed around 'nil' is 'SENILE'.
'being' is the link.
(Other definitions for senile that I've seen before include "Old and infirm" , "In one's dotage" , "Mentally weak due to old age" , "past it" , "Suffering from mental weakness due to old age" .)