Galilean satellite first penetrating rings (8)
I believe the answer is:
callisto
'galilean satellite' is the definition.
'callisto' can be an answer for 'satellite' (I've seen this before). I'm unsure of the 'galilean' bit.
'first penetrating rings' is the wordplay.
'first' becomes 'ist'.
'penetrating' indicates putting letters inside.
'rings' becomes 'callo' (I can't explain this - if you can you should give a lot more credence to this answer).
'ist' placed within 'callo' is 'CALLISTO'.
(Other definitions for callisto that I've seen before include "moon of Jupiter" , "Female reformed to bear" , "nymph revealing > heavenly body" , "satellite" .)