Go slow, entering sunbeam -- this doesn't happen often (6)
I believe the answer is:
rarity
'this doesn't happen often' is the definition.
The definition suggests a singular noun which matches the answer.
'go slow entering sunbeam' is the wordplay.
'go slow' becomes 'rit' (I can't explain this - if you can you should believe this answer much more).
'entering' indicates putting letters inside.
'sunbeam' becomes 'ray' (I've seen this in another clue).
'rit' going into 'ray' is 'RARITY'.
Can you help me to learn more?
(Other definitions for rarity that I've seen before include "It occurs infrequently" , "Very unusual occurrence" , "I tarry (anag.)" , "Thinness" , "Something very scarce and often valuable" .)