Son meeting snake is not so happy (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')
'son meeting snake' is the wordplay.
'son' becomes 's'.
'meeting' is a charade indicator (letters next to each other).
'snake' becomes 'adder' (adder is a kind of snake).
's'+'adder'='SADDER'
'is' acts as a link.
(Other definitions for sadder that I've seen before include "Unhappier" , "Less happy" , "more blue" , "extra-blue?" , "Gloomier" .)