Colour of pests digesting crude oil? (9)
I believe the answer is:
vermilion
'colour' is the definition.
(vermilion is a kind of colour)
'pests digesting crude oil?' is the wordplay.
'pests' becomes 'vermin' (I've seen this before).
'digesting' means one lot of letters goes inside another.
'crude' is an anagram indicator.
'oil' anagrammed gives 'lio'.
'vermin' placed around 'lio' is 'VERMILION'.
'of' acts as a link.
(Other definitions for vermilion that I've seen before include "Ron, I'm live with a bright red pigment" , "Bright red colour" , "Bright scarlet" , "Brilliant red" , "shade" .)