Unlikely to add up if short (4)
I believe the answer is:
tall
'unlikely' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for tall that I've seen before include "High in stature" , "Higher than the average person" , "Above average height" , "Unbelievable" , "Towering" .)