Unlikely score Yankee discounted (4)
I believe the answer is:
tall
'unlikely' is the definition.
(I've seen this in other clues)
'score yankee discounted' is the wordplay.
'score' becomes 'tally' (tally is a kind of score).
'yankee' becomes 'y' (phonetic alphabet: alpha, bravo, charlie etc.).
'discounted' indicates named letters should be taken away.
'tally' with 'y' removed is 'TALL'.
(Other definitions for tall that I've seen before include "Like giants" , "Higher than the average person" , "Unbelievable" , "description of a giraffe?" , "High in stature" .)