Lobster atop coloured stick (6)
I believe the answer is:
'coloured stick' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'lobster atop' is the wordplay.
'lobster' becomes 'cray' (I can't explain this - if you can you should believe this answer much more).
'atop' becomes 'on'.
Can you help me to learn more?
(Other definitions for crayon that I've seen before include "alternative to charcoal" , "Coloured wax pencil" , "Drawing-stick" , "Pencil of chalk or wax" , "Drawing pen" .)