Substance of planet before sun — Mercury, say? (4,4)
I believe the answer is:
rock star
'mercury say?' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'substance of planet before sun' is the wordplay.
I cannot really see how this works, but
'substance' could be 'rock' (rock is a kind of substance) and 'rock' is located in the answer.
'sun' could be 'star' (I've seen this before) and 'star' is present in the answer.
No letters remain.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rock star that I've seen before include "Pop idol" , "Very famous pop singer" , "Bono, say" , "Mick Jagger, say" , "Famous musician" .)