Muppet, classic character ends in film clip: Kermit say? (6)
I believe the answer is:
numpty
'muppet' is the definition.
'classic character ends in film clip kermit say?' is the wordplay.
'classic character' becomes 'nu' (letter in modern and ancient Greek).
'ends in' says to take the final letters (I've seen 'end in' mean this).
The final letters of 'film clip kermit say' are 'mpty'.
'nu'+'mpty'='NUMPTY'
(Other definitions for numpty that I've seen before include "idiot from way up here" , "Fool" , "A Scottish idiot" .)