Improve creed - mine is rickety (8)
I believe the answer is:
decrepit
'is rickety' is the definition.
(I've seen this before)
'improve creed mine' is the wordplay.
'improve' indicates an anagram.
'mine' becomes 'pit' (down the pit).
'creed' with letters rearranged gives 'decre'.
'decre'+'pit'='DECREPIT'
(Other definitions for decrepit that I've seen before include "Frail and creaky" , "Dilapidated, made weak by age" , "Worn and broken down by use" , "Run-down by old age or hard use" , "Pet cried (anag) - worn out" .)