Weakness of old age (8)
I believe the answer is:
senility
'weakness of old age' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for senility that I've seen before include "Dotage" , "State of bodily or mental weaknes due to age" , "Difficulty getting on" , "Mental weakness due to old age" , "Mental infirmity of old age" .)