It’s after 51 thousand that there are restrictions (6)
I believe the answer is:
limits
'there are restrictions' is the definition.
I can't judge whether this definition defines the answer.
'it's after 51 thousand' is the wordplay.
'after' means one lot of letters go next to another.
'51' becomes 'li' (Roman numerals).
'thousand' becomes 'm' ('m' can be a synonym of 'thousand').
'li'+'m'='lim'
'its' after 'lim' is 'LIMITS'.
'that' acts as a link.
Can you help me to learn more?
(Other definitions for limits that I've seen before include "Boundaries, confines" , "Biblical prophet" , "Points beyond which one may not go" , "Boundaries or furthest reaches" , "Boundaries, points beyond which something cannot continue" .)