Small snake more depressed (6)
I believe the answer is:
sadder
'more depressed' is the definition.
(I've seen this in another clue)
'small snake' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'snake' becomes 'adder' (type of snake).
's'+'adder'='SADDER'
(Other definitions for sadder that I've seen before include "Unhappier" , "Lower" , "Less happy" , "more blue" , "More unhappy (but hopefully 1 down)" .)