Not so happy, seeing small snake (6)
I believe the answer is:
sadder
'not so happy' is the definition.
('sad' can be the opposite of 'happy' so perhaps 'sadder' is reasonable for 'not so happy')
'small snake' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'snake' becomes 'adder' (adder is a kind of snake).
's'+'adder'='SADDER'
'seeing' acts as a link.
(Other definitions for sadder that I've seen before include "More unhappy (but hopefully 1 down)" , "Lower" , "extra-blue?" , "Gloomier" , "More downcast" .)