Conservationist of a sickly hue (5)
I believe the answer is:
green
This might be a double definition.
'conservationist' is the first definition.
(I've seen this before)
'sickly hue' is the second definition.
'green' can be an answer for 'hue' (green is a kind of colour). I'm not certain of the 'sickly' bit.
'of a' acts as a link.
Can you help me to learn more?
(Other definitions for green that I've seen before include "Lacking experience (on the golf course?)" , "cheap marijuana" , "Environmentally sound" , "The colour of inexperience" , "Proverbial constituent of our moon" .)