Emaciated Yankee just below summit (5)
I believe the answer is:
peaky
'emaciated' is the definition.
(thesaurus)
'yankee just below summit' is the wordplay.
'yankee just' becomes 'y' (I can't justify this - if you can you should give a lot more credence to this answer).
'below' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'summit' becomes 'peak' (mountain peak).
'y' put after 'peak' is 'PEAKY'.
Can you help me to learn more?
(Other definitions for peaky that I've seen before include "Slightly unwell" , "Seedy" , "Rather sickly" .)