Outing led astray by chart marking (9)
I believe the answer is:
longitude
'chart marking' is the definition.
'longitude' can be an answer for 'chart' (thesaurus). I am not sure about the 'marking' bit.
'outing led astray' is the wordplay.
'astray' indicates anagramming the letters.
'outing'+'led'='outingled'
'outingled' anagrammed gives 'LONGITUDE'.
'by' is the link.
Can you help me to learn more?
(Other definitions for longitude that I've seen before include "Distance in degrees" , "Distance east or west of the Greenwich meridian" , "Length" , "this helps define our position" , "Word often paired with latitude" .)