'Banish' lisped is terrible (6)
I believe the answer is:
dispel
'banish' is the definition.
(I've seen this before)
'lisped is terrible' is the wordplay.
'is terrible' indicates anagramming the letters.
'lisped' anagrammed gives 'DISPEL'.
(Other definitions for dispel that I've seen before include "Get rid of" , "Banish (from the mind)" , "Dissipate or drive off" , "Make (something) disappear" , "Blow away, scatter" .)