Restrict it after 51,000 (5)
I believe the answer is:
limit
'restrict' is the definition.
(I know that restrict can be written as limit)
'it after 51000' is the wordplay.
'after' is a charade indicator (letters next to each other).
'5l000' becomes 'lim' (I can't explain this - if you can you should believe this answer much more).
'it' put after 'lim' is 'LIMIT'.
Can you help me to learn more?
(Other definitions for limit that I've seen before include "qualify?" , "Cutoff point" , "Point beyond which one should not go" , "verge" , "end" .)