It contributes colourfully to an illusion (4)

I believe the answer is:

anil

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'it contributes colourfully' is the definition.
The definition suggests a singular noun which matches the answer.

'an illusion' is the wordplay.
'illusion' becomes 'il' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'an'+'il'='ANIL'

'to' is the link.

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(Other definitions for anil that I've seen before include "Indigo (plant or dye)" , "A blue dye" , "Colouring" , "Shrub providing indigo" , "Indian man" .)

I've seen this clue in the Evening Standard.

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