Head of smooth snake is more blue (6)
I believe the answer is:
sadder
'more blue' is the definition.
('sad' can be similar in meaning to 'blue')
'head of smooth snake' is the wordplay.
'head of' indicates taking the first letters.
'snake' becomes 'adder' (adder is a kind of snake).
The initial letter of 'smooth' is 's'.
's'+'adder'='SADDER'
'is' acts as a link.
(Other definitions for sadder that I've seen before include "Unhappier" , "More unhappy (but hopefully 1 down)" , "Gloomier" , "More downcast" , "Less happy" .)