Best course time in past achievements (5,6)
I believe the answer is:
track record
'past achievements' is the definition.
(I've seen this before)
'best course time' is the wordplay.
I cannot really understand how this works, but
'best' could be 'record' (similar in meaning) and 'record' is located in the answer.
'course' could be 'track' (track is a kind of course) and 'track' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'in' acts as a link.
Can you help me to learn more?
(Other definitions for track record that I've seen before include "Past account" , "Background" , "List of past achievements and failures" , "Sum of recognised accomplishments" , "History" .)