It's horizontal, honest (2,3,5)
I believe the answer is:
on the level
'honest' is the definition.
(I've seen this in another clue)
'it's horizontal' is the wordplay.
I cannot really understand how this works, but
'horizontal' could be 'level' (similar in meaning) and 'level' is found in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for on the level that I've seen before include "Fair; honestly speaking" , "Honest - genuine" , "No tipping" , "Honest, truthful (colloq.)" , "Some horse racing is" .)