Maintain flat surface not necessary (9)
I believe the answer is:
avertable
'not necessary' is the definition.
(something avertable doesn't have to happen)
'maintain flat surface' is the wordplay.
'maintain' becomes 'aver' (averring is a kind of maintaining).
'flat surface' becomes 'table' (I've seen this before).
'aver'+'table'='AVERTABLE'
(Other definitions for avertable that I've seen before include "may not come into play?" , "That's not bound to happen" .)