In good condition, having accumulated fine years in several decades (5)
I believe the answer is:
fifty
'in good condition having accumulated fine years in several decades' is the definition.
I can't judge whether this defines the answer.
'in good condition having accumulated fine years' is the wordplay.
'in good condition' becomes 'fit' (I've seen this in another clue).
'having' is an insertion indicator.
'accumulated fine' becomes 'f' (abbreviation in pencil classifications. I am not sure about the 'accumulated' bit.).
'years' becomes 'y' (abbreviation).
'fit' placed around 'f' is 'fift'.
'fift'+'y'='FIFTY'
Can you help me to learn more?
(Other definitions for fifty that I've seen before include "Half-century" , "A multiple of ten" , "L" .)