Let it be known indirectly there are no wagons in Washington (4)
I believe the answer is:
hint
'known indirectly there are no wagons in washington' is the definition.
I can't judge whether this defines the answer.
'let it' is the wordplay.
'let' becomes 'hin' (I can't justify this - if you can you should believe this answer much more).
'it' becomes 't' (abbreviation. e.g. in 'tis).
'hin'+'t'='HINT'
'be' acts as a link.
Can you help me to learn more?
(Other definitions for hint that I've seen before include "A dash" , "Indirect clue" , "Helpful suggestion" , "Clue - soupcon" , "Tip - small amount" .)