Observe nothing in being old and infirm (6)
I believe the answer is:
senile
'old and infirm' is the definition.
(I've seen this before)
'observe nothing in' is the wordplay.
'observe' becomes 'see' (I've seen this before).
'nothing' becomes 'nil' ('nil' can be a synonym of 'nothing').
'in' is an insertion indicator.
'see' placed around 'nil' is 'SENILE'.
'being' is the link.
(Other definitions for senile that I've seen before include "Doddering" , "In one's dotage" , "Mentally infirm with age" , "Affected by old age" , "Old and doddery" .)