Improbable (8)
'improbable' is the definition.
(I know that improbable can be written as unlikely)
This is all the 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" .)