What the police had on his hair? (3)
I believe the answer is:
oil
'what the police had on his hair?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
This is all the clue.
Can you help me to learn more?
(Other definitions for oil that I've seen before include "what can be seen on iceberg" , "Lubricate; fuel" , "painting" , "Liquid fuel" , "Fossil fuel" .)