Like Molly Brown with a lisp? Highly unlikely (11)
I believe the answer is:
unthinkable
'unlikely' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for unthinkable that I've seen before include "it's out of the question" , "Too unlikely to consider" , "Inconceivable" , "Preposterous" .)