List where thousands finish within the two top grades (6)
I believe the answer is:
agenda
'list' is the definition.
(agenda is a kind of list)
'finish within the two top grades' is the wordplay.
'finish' becomes 'end' (synonyms).
'within' indicates putting letters inside.
'the two top grades' becomes 'aga' (I can't explain this - if you can you should give a lot more credence to this answer).
'end' inserted into 'aga' is 'AGENDA'.
'where thousands' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for agenda that I've seen before include "List of items for committee to consider" , "Scheme" , "Item list for meeting" , "Items for discussion at committee meeting" , "Business to be transacted" .)