Scales to go up: Yankee is heavy (6)
I believe the answer is:
sleepy
'heavy' is the definition.
(thesaurus)
'scales to go up yankee' is the wordplay.
'scales' becomes 'sl' (I can't explain this - if you can you should believe this answer much more).
'to go' becomes 'pee' (both can mean to urinate).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'pee' reversed gives 'eep'.
'sl'+'eep'+'y'='SLEEPY'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for sleepy that I've seen before include "Rather tired" , "Lethargic" , "Feeling like having a snooze" , "Doc's nodding pal" , "(Of place) low in activity" .)