Spot mature time of feeble-mindedness (6)
I believe the answer is:
dotage
'time of feeble-mindedness' is the definition.
(time of life when one is weak or feeble)
'spot mature' is the wordplay.
'spot' becomes 'dot' (I've seen this before).
'mature' becomes 'age' (synonyms).
'dot'+'age'='DOTAGE'
(Other definitions for dotage that I've seen before include "Foolish later life" , "decrepitude" , "Declining years" , "State of weakness of mind caused by old age" , "Senile years" .)