Fifty is under forty-nine? Well, no! (3)
I believe the answer is:
ill
'well no' is the definition.
Both the answer and definition are adjectives. Perhaps there's a link between them I don't understand?
'fifty is under forty-nine?' is the wordplay.
'fifty' becomes 'l' (Roman numeral).
'is under' says to put letters next to each other (in a down clue, some letters go below others).
'forty nine?' becomes 'il' (I can't explain this - if you can you should believe this answer much more).
'l' put after 'il' is 'ILL'.
Can you help me to learn more?
(Other definitions for ill that I've seen before include "Indisposed" , "Seasick" , "Not well, sick" , "Wicked" , "Imperfectly; unwell" .)