Against supporting the rise of coloured diesel (4)
I believe the answer is:
derv
'diesel' is the definition.
(I've seen this before)
'against supporting the rise of coloured' is the wordplay.
'against' becomes 'v' (abbreviation for 'versus').
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'the rise of' shows that the letters should be reversed in order.
'coloured' becomes 'red' ('red' can be similar in meaning to 'coloured').
'red' back-to-front is 'der'.
'v' after 'der' is 'DERV'.
(Other definitions for derv that I've seen before include "Type of fuel" , "Fuel for lorries" , "Water monster" , "Vehicle fuel oil" , "Vehicle diesel-oil" .)