Crossword Genius

Rainbow possibly mysterious (4)

Ross

I believe the answer is:

arch

I'm a little stuck... Click here to teach me more about this clue!

'mysterious' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I can't see?

'rainbow possibly' is the wordplay.
'rainbow' becomes 'arc' (I've seen this before).
'possibly' becomes 'h' (I can't justify this - if you can you should give a lot more credence to this answer).
'arc'+'h'='ARCH'

Can you help me to learn more?

(Other definitions for arch that I've seen before include "Curved spanning structure" , "Curved span" , "Curved under-part of foot" , "Vaulted opening" , "Curve spanning an opening" .)

I've seen this clue in The Telegraph.
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