What the poilu has on his hair? (3)
I believe the answer is:
oil
'his hair?' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'what the poilu has' is the wordplay.
'has' indicates a hidden word.
'OIL' can be found hidden inside 'what poilu'.
'on' acts as a link.
Can you help me to learn more?
(Other definitions for oil that I've seen before include "painting" , "What can be extracted" , "Fossil fuel" , "what can be seen on iceberg" , "Thick liquid that doesn't dissolve in water" .)