Rule requiring rock to be stored in the mine's entrance (7)
I believe the answer is:
theorem
'rule' is the definition.
(I have seen 'Mathematical rule ' mean 'theorem' so perhaps 'rule' could also mean 'theorem')
'rock to be stored in the mine's entrance' is the wordplay.
'rock' becomes 'ore' (I can't explain this - if you can you should believe this answer much more).
'to be stored in' means one lot of letters goes inside another.
'entrance' indicates taking the first letters.
The initial letter of 'mines' is 'm'.
'the'+'m'='them'
'ore' going inside 'them' is 'THEOREM'.
'requiring' acts as a link.
Can you help me to learn more?
(Other definitions for theorem that I've seen before include "Proven hypothesis" , "Mathematical formula" , "(Maths) proposition" , "Mathematical statement to be proved" , "Hero met (anag) - principle that's proven" .)