'Earth and Mars Rocks' by American scholar (7)
I believe the answer is:
erasmus
'scholar' is the definition.
(Erasmus is an example)
'earth and mars rocks by american' is the wordplay.
'earth' becomes 'E'.
'and' is a charade indicator (letters next to each other).
'rocks' indicates an anagram.
'by' is a charade indicator (letters next to each other).
'american' becomes 'us' (abbreviation for United States).
'mars' anagrammed gives 'rasm'.
'e'+'rasm'+'us'='ERASMUS'
(Other definitions for erasmus that I've seen before include "Learned person" , "religious scholar" , "Renaissance scholar" , "Dutch mediaeval humanist" , "Desiderius -, fifteenth and sixteenth century Dutch humanist" .)