Mental decline (6)
I believe the answer is:
dotage
'mental decline' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for dotage that I've seen before include "Foolish later life" , "Senile stage" , "when one's powers decline" , "Feeble-minded period late in life" , "Foolish infatuation" .)