Elderly and infirm agent wearing wrong ID, etc. (8)
I believe the answer is:
decrepit
'elderly and infirm' is the definition.
(I've seen this before)
'agent wearing wrong id etc' is the wordplay.
'agent' becomes 'rep' (short for representative).
'wearing' indicates putting letters inside.
'wrong' is an anagram indicator.
'id'+'etc'='idetc'
'idetc' with letters rearranged gives 'decit'.
'rep' placed inside 'decit' is 'DECREPIT'.
(Other definitions for decrepit that I've seen before include "Depicter may be weakened by old age" , "Dilapidated, made weak by age" , "Old and feeble" , "rickety" , "ruined by neglect" .)