For you it's hot, in a little cup (4)
I believe the answer is:
thee
'for you' is the definition.
(I've seen this before)
'hot in a little cup' is the wordplay.
'hot' becomes 'h' (abbreviation eg on taps).
'in' means one lot of letters goes inside another.
'a little cup' becomes 'tee' (I can't justify this - if you can you should give a lot more credence to this answer).
'h' inserted inside 'tee' is 'THEE'.
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for thee that I've seen before include "Poetically you" , "Archaic form of 'you'" , "Old objective second person singular" , "You (arch.)" , "you many years ago" .)