Dropping back, dismiss a total flipping idiot (9)
I believe the answer is:
ignoramus
'idiot' is the definition.
(I've seen this before)
'dropping back dismiss a total flipping' is the wordplay.
'dropping back' means to remove the last letter.
'dismiss' becomes 'ignore' ('ignore' can be a synonym of 'dismiss').
'total' becomes 'sum' ('sum' can be a synonym of 'total').
'flipping' says the letters should be written backwards.
'sum' written backwards gives 'mus'.
'ignore' with its last letter taken off is 'ignor'.
'ignor'+'a'+'mus'='IGNORAMUS'
(Other definitions for ignoramus that I've seen before include "unwitting person" , "one in a state of bliss?" , "Ignorant person" , "Ron is a mug (anag.)" , "One is not intelligent" .)