Blue voter against Conservative sent up (4)
I believe the answer is:
cyan
'blue' is the definition.
(I have seen 'Greenish blue ' mean 'cyan' so perhaps 'blue' could also mean 'cyan')
'voter against conservative sent up' is the wordplay.
'voter' becomes 'nay' (I can't explain this - if you can you should give a lot more credence to this answer).
'against' is a charade indicator (letters next to each other).
'conservative' becomes 'C' (UK political abbreviation).
'sent up' is a reversal indicator.
'nay'+'c'='nayc'
'nayc' reversed gives 'CYAN'.
Can you help me to learn more?
(Other definitions for cyan that I've seen before include "Primary blue colour in printing" , "Greenish-blue colour complementary to red" , "Blue-green colour" , "Bluish green" , "Greeny blue" .)