Rule broken by unknown character in valley (6)
I believe the answer is:
'valley' is the definition.
(I know that canyon is a type of valley)
'rule broken by unknown character' is the wordplay.
'rule' becomes 'canon' (canon can mean a law).
'broken by' means one lot of letters goes inside another (inserted letters break into the word).
'unknown character' becomes 'y' (algebra).
'canon' placed around 'y' is 'CANYON'.
'in' acts as a link.
(Other definitions for canyon that I've seen before include "Gulch" , "Narrow steep-sided valley" , "Big rift" , "deep container for water" , "The Grand . . . . . . of Colorado is a natural phenomenon in USA" .)