Thin? Judge in the Sun's issue (6)
I believe the answer is:
'thin?' is the definition.
(I've seen this before)
'judge in the sun's issue' is the wordplay.
'judge' becomes 'ref' (referee).
'in the' indicates putting letters inside.
'sun's issue' becomes 'ray' (I can't explain this - if you can you should give a lot more credence to this answer).
'ref' placed inside 'ray' is 'RAREFY'.
Can you help me to learn more?
(Other definitions for rarefy that I've seen before include "Fry are (anag.)" , "To make less dense" , "Become less dense" , "Make more refined" , "Make less solid; a ferry (anag.)" .)