Deny as to punish? Be reasonable! (6,2)
I believe the answer is:
steady on
'be reasonable' is the definition.
(phrase used to ask someone to be reasonable)
'deny as to punish?' is the wordplay.
'punish?' indicates anagramming the letters (I've seen 'punished' mean this).
'deny'+'as'+'to'='denyasto'
'denyasto' is an anagram of 'STEADY ON!'.
(Other definitions for steady on that I've seen before include "That's a bit much!" , "Be careful" , "Be reasonable" , "Don't be so hasty" , "No set day (anag) - keep calm!" .)