Turns nasty colour, repulsed by sulphur (8)
I believe the answer is:
meanders
'turns nasty colour repulsed by sulphur' is the definition.
I can't judge whether this defines the answer.
'turns nasty colour repulsed by sulphur' is the wordplay.
'turns nasty' becomes 'mean' (I can't justify this - if you can you should give a lot more credence to this answer).
'colour' becomes 'red' (red is a kind of colour).
'repulsed' says the letters should be written in reverse.
'by' means one lot of letters go next to another.
'sulphur' becomes 'S' (synonyms).
'red' in reverse letter order is 'der'.
'mean'+'der'+'s'='MEANDERS'
Can you help me to learn more?
(Other definitions for meanders that I've seen before include "Rambles aimlessly on a winding course" , "(Of river) winds" , "Wanders about with no fixed destination" , "Wanders in leisurely, winding way" , "Wanders randomly" .)