Log an unsurpassed achievement (6)
I believe the answer is:
record
This clue might be a double definition.
'log' is the first definition.
(I know that log is a kind of record)
'an unsurpassed achievement' is the second definition.
'record' can be an answer for 'achievement' (record is a kind of achievement). I am unsure of the 'an unsurpassed' bit.
Can you help me to learn more?
(Other definitions for record that I've seen before include "Chronicle or log" , "Tape" , "See 21" , "Written account (none better!)" , "Set down in permanent form" .)