He is a colourless one (7)
I believe the answer is:
element
'he' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Maybe there's a link between them I don't understand?
'colourless one' is the wordplay.
I cannot really understand how this works, but
'one' could be 'me' (a setter might use 'one' to humourously mean 'me') and 'me' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for element that I've seen before include "Iron, for instance - part of an electric kettle" , "fire, perhaps, once" , "that's used in heating" , "Fire (or part of one)" , "Eg copper" .)