Being decrepit, check out nothing (6)
I believe the answer is:
senile
'being decrepit' is the definition.
(I've seen this in another clue)
'check out nothing' is the wordplay.
'check' becomes 'see' (synonyms).
'out' means one lot of letters goes inside another.
'nothing' becomes 'nil' (eg nil is zero in football).
'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" .)