Highly unlikely (4)
I believe the answer is:
tall
'unlikely' is the definition.
(as in a tall tale)
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 "description of a giraffe?" , "Remarkable" , "Above average height" , "hard to believe in" , "Vertically large" .)