Speak falsely to stop an outsider (5)
I believe the answer is:
alien
'outsider' is the definition.
(I've seen this in another clue)
'speak falsely to stop an' is the wordplay.
'speak falsely' becomes 'lie' (I can't justify this - if you can you should believe this answer much more).
'to stop' means one lot of letters goes inside another.
'lie' placed within 'an' is 'ALIEN'.
Can you help me to learn more?
(Other definitions for alien that I've seen before include "24a [ESTRANGE]" , "Martian, say" , "coming from overseas" , "Foreign, strange" , "Foreign - film directed by Ridley Scott" .)