Rainbow possibly mysterious (4)
I believe the answer is:
arch
'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" .)