... knocking around at nine hundred like Methuselah (7)
I believe the answer is:
ancient
'knocking around at nine hundred like methuselah' is the definition.
I can't judge whether this defines the answer.
'knocking around at nine' is the wordplay.
I cannot really see how this works, but
'around' could be 'c' (abbreviation for 'circa') and 'c' is located in the answer.
an anagram of 'nine' is 'nien' which is located in the left over letters.
'at' is within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for ancient that I've seen before include "Very old, like the poetic mariner" , "Antediluvian" , "of long standing" , "Antiquated" , "Early" .)