Being decrepit, check out nothing (6)
I believe the answer is:
senile
'being decrepit' is the definition.
(thesaurus)
'check out nothing' is the wordplay.
'check' becomes 'see' ('see' can be a synonym of 'check').
'out' means one lot of letters goes inside another.
'nothing' becomes 'nil' ('nil' can be a synonym of 'nothing').
'see' placed around 'nil' is 'SENILE'.
(Other definitions for senile that I've seen before include "past it" , "Suffering from mental weakness due to old age" , "Affected by old age" , "Mentally infirm with age" , "Suffering from old age's weaknesses" .)