As far as I can see, story mostly is on the level (10)
I believe the answer is:
horizontal
'on the level' is the definition.
(flat or level)
'as far as i can see story mostly' is the wordplay.
'as far as i can see' becomes 'horizon' (you can't see beyond the horizon).
'story' becomes 'tale' (synonyms).
'mostly' means to remove the last letter.
'tale' with its last letter taken away is 'tal'.
'horizon'+'tal'='HORIZONTAL'
'is' acts as a link.
(Other definitions for horizontal that I've seen before include "Lying flat" , "What this answer isn't!" , "even" , "Prone" .)