As a general rule, said to be at the top (9)
I believe the answer is:
principle
'the top' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'as a general rule said to be' is the wordplay.
I cannot really see how this works, but
'as' could be 'in' (I've seen this in other clues) and 'in' is found in the answer.
'rule' could be 'r' (legal abbreviation) and 'r' is found in the answer.
This explanation may well be incorrect...
'at' is the link.
Can you help me to learn more?
(Other definitions for principle that I've seen before include "Morally correct behaviour" , "Law of nature, rule of conduct" , "Tenet" , "In which, theoretically" , "honour" .)