When blue, it flies (6)
I believe the answer is:
bottle
This could be a double definition.
'when blue' is the first definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's a link between them I don't understand?
'it flies' is the second definition.
The definition suggests a singular noun which matches the answer.
Can you help me to learn more?
(Other definitions for bottle that I've seen before include "confidence" , "Narrow-necked vessel" , "Glass or plastic container" , "Grit" , "beer found in this?" .)