Silent movie star is to take a tongue-tied part (6)
I believe the answer is:
keaton
'silent movie star' is the definition.
'take a tongue-tied part' is the wordplay.
'part' indicates the answer is hidden within the clue.
'KEATON' is hidden within 'take a tongue tied'.
'is to' is the link.
(Other definitions for keaton that I've seen before include "'Buster' of the silent films" , "Buster -, US film actor" , "Diane, Michael or Buster!" , "Buster -, US film comedian" , "Buster ...... was star comic of silent screen" .)