Wave moving right to left would be flatter (8)
I believe the answer is:
brandish
'wave' is the definition.
(I know that wave can be written as brandish)
'moving right to left would be flatter' is the wordplay.
I cannot really see how this works, but
'right' could be 'r' (common abbreviation) and 'r' is found in the answer.
'would' could be 'd' (I've seen this in other clues) and 'd' is found within the answer.
The remaining letters 'banish' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for brandish that I've seen before include "Wave something about threateningly" , "Flourish (a weapon)" , "Wave like a weapon" , "Exhibit aggressively" , "Wield (weapon)" .)