Improbable (8)
'improbable' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for unlikely that I've seen before include "Not expected to happen" , "Remote" , "Not to be expected, don't bet on it" , "Not probable, really the reverse" , "Implausible" .)
(Other definitions for slim that I've seen before include "Bakery produce" , "Thin; WWtwo Burma commander" , "(Of chance) very slight" , "Slender and elegant" , "insufficient" .)
(Other definitions for far-fetched that I've seen before include "Forced - unconvincing" , "Incredible" , "(Tale) hard to believe" , "Highly implausible" , "Unlikely" .)