Highly improbable (4)
I believe the answer is:
tall
'improbable' is the definition.
(synonyms)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for tall that I've seen before include "Towering" , "Bigger than average" , "Remarkable" , "Of great height" , "Unbelievable" .)