### Clue: It's essential to follow clues by directions (8)
### Answer: KEYSTONE 
 ### Explanation: Definition: It's essential
 TO **placed after**(follow) KEYS(clues;things that helps to one to understand something, eg. the legend in a map) **plus**(by) N,E(abbrev. for "north" and "east";directions).
One might argue it should be "follows" in this instance, but that would make the surface ungrammatical.
It didn't help that "key" also means "essential".