Unlikely score Yankee discounted (4)
I believe the answer is:
Here is my best explanation:
'unlikely' is the definition.
(I've seen this in other clues)
'score yankee discounted' is the subsidiary indication.
'score' becomes 'tally' (tally is a kind of score).
'yankee' becomes 'y' (phonetic alphabet: alpha, bravo, charlie etc.).
'discounted' suggests deleting specific letters.
'tally' with 'y' taken out is 'tall'.
(Other definitions for tall that I've seen before include "that's hard to swallow", "Unlikely", "Of great height", "Unbelievable", "Towering", "High in stature", "Above average height", "Like giants", "Bigger than average" and "Higher than the average person".)