Best seats here, but a thousand can’t sit down? (10)
I believe the answer is:
grandstand
'here but a thousand can't sit down?' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'best seats' is the wordplay.
'best' becomes 'grand' (I can't justify this - if you can you should believe this answer much more).
'seats' becomes 'stand' (stand is a kind of seats).
'grand'+'stand'='GRANDSTAND'
Can you help me to learn more?
(Other definitions for grandstand that I've seen before include "Racecourse construction" , "Main structure for watching sporting events" , "Racecourse building" , "Terraced block of seats commanding the best view of a sporting event" , "Racecourse structure" .)