Up-front with regard to the plane (2,3,5)
I believe the answer is:
on the level
'up-front' is the definition.
(I've seen this in another clue)
'regard to the plane' is the wordplay.
'regard to' becomes 'on' (I can't explain this - if you can you should give a lot more credence to this answer).
'plane' becomes 'level' (synonyms).
'on'+'the'+'level'='ON THE LEVEL'
'with' acts as a link.
Can you help me to learn more?
(Other definitions for on the level that I've seen before include "Some horse racing is" , "No tipping" , "Honest, truthful (colloq.)" , "Honest - genuine" , "Fair; honestly speaking" .)