It's well known in astronomical circles (6)
I believe the answer is:
saturn
'it's well known in astronomical circles' is the definition.
I can't judge whether this definition defines the answer.
'it's well known in astronomical circles' is the wordplay.
'it' becomes 'sa' (old-fashioned abbreviation for sex appeal).
'well known in astronomical circles' becomes 'turn' (I can't explain this - if you can you should give a lot more credence to this answer).
'sa'+'turn'='SATURN'
Can you help me to learn more?
(Other definitions for saturn that I've seen before include "One of the 1 across" , "Roman god - planet" , "Sixth rock from the sun" , "The dark planet" , "Titan its moon" .)