More blue markings at the bottom on reptile (6)
I believe the answer is:
sadder
'more blue' is the definition.
('sad' can be similar in meaning to 'blue')
'markings at the bottom on reptile' is the wordplay.
'at the bottom' suggests the final letters.
'on' says to put letters next to each other.
'reptile' becomes 'adder' (adder is a kind of reptile).
The final letter of 'markings' is 's'.
's'+'adder'='SADDER'
(Other definitions for sadder that I've seen before include "Lower" , "Unhappier" , "More downcast" , "Gloomier" , "extra-blue?" .)