To be honest, it's flat (5)
I believe the answer is:
level
This clue is a double definition.
'to be honest' is the first definition.
(level can mean fair or honest)
'flat' is the second definition.
(I know that flat can be written as level)
'it's' acts as a link.
(Other definitions for level that I've seen before include "Palindromically smooth" , "Straight, even" , "Not in front or behind" , "Device ot establish a flat surface" , "Neither winning, nor losing" .)