Truthful in flat (2,3,5)
I believe the answer is:
on the level
'truthful' is the definition.
(I've seen this before)
'flat' is the wordplay.
I cannot really see how this works, but
'flat' could be 'level' ('level' can be a synonym of 'flat') and 'level' is located in the answer.
This may be the basis of clue (or it may be nonsense).
'in' acts as a link.
Can you help me to learn more?
(Other definitions for on the level that I've seen before include "No tipping" , "Fair; honestly speaking" , "Honest, truthful (colloq.)" , "Honest - genuine" , "Some horse racing is" .)