For once, behave right (5)
I believe the answer is:
exact
'right' is the definition.
(similar in meaning)
'for once behave' is the wordplay.
I cannot really see how this works, but
'once' could be 'ex' (prefix meaning former or onetime) and 'ex' is found in the answer.
'behave' could be 'act' (as in acting or behaving a certain way) and 'act' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for exact that I've seen before include "Correct; insist on" , "Particular" , "On the nail" , "Precisely accurate" , "on the money" .)