Bluer tail of grass snake (6)
I believe the answer is:
sadder
'bluer' is the definition.
('sad' can be similar in meaning to 'blue')
'tail of grass snake' is the wordplay.
'tail of' says to take the final letters.
'snake' becomes 'adder' (type of snake).
The final letter of 'grass' is 's'.
's'+'adder'='SADDER'
(Other definitions for sadder that I've seen before include "Unhappier" , "more blue" , "Gloomier" , "extra-blue?" , "More unhappy (but hopefully 1 down)" .)