Crossword Genius

Fight dirty (4)

Ross

I believe the answer is:

blue

I'm a little stuck... Click here to teach me more about this clue!

This might be a double definition.

'fight' is the first definition.
The answer and definition can be both human attributes as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

'dirty' is the second definition.
(blue can mean pornographic)

Can you help me to learn more?

(Other definitions for blue that I've seen before include "Spend all at once - colour" , "gloomy" , "Colour - smutty" , "Terrible commotion?" , "Oxbridge sports award" .)

I've seen this clue in the Sydney Morning Herald.
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