Dither insanely about a good period for retirement activity (5,3)
I believe the answer is:
third age
'period for retirement activity' is the definition.
(I've seen this before)
'dither insanely about a good' is the wordplay.
'insanely' indicates an anagram.
'about' means one lot of letters goes inside another.
'good' becomes 'g' (abbreviation).
'dither' is an anagram of 'thirde'.
'a'+'g'='ag'
'thirde' enclosing 'ag' is 'THIRD AGE'.
(Other definitions for third age that I've seen before include "period for retirement activity" , "Old people collectively" , "elderly group" .)