Happy with small cup (4)
I believe the answer is:
smug
'happy' is the definition.
Both the definition and answer are adjectives. Perhaps you can see a link between them that I can't see?
'with small cup' is the wordplay.
'with' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cup' becomes 'mug' (I've seen this before).
's' put next to 'mug' is 'SMUG'.
Can you help me to learn more?
(Other definitions for smug that I've seen before include "Self-satisfied, stuffy" , "Spruce" , "Irritatingly pleased with oneself" , "Self-satisfied about gums" , "Prim" .)