English novice coming up to measure exotic rock (9)
I believe the answer is:
meteorite
'exotic rock' is the definition.
(I've seen this in another clue)
'english novice coming up to measure' is the wordplay.
'english' becomes 'E' (abbreviation).
'novice' becomes 'tiro' (synonyms).
'coming up' is a reversal indicator (in a down clue, letters go up).
'to' means one lot of letters go next to another (I've seen this in other clues).
'measure' becomes 'mete' (meting things out is measuring them out).
'e'+'tiro'='etiro'
'etiro' in reverse letter order is 'orite'.
'orite' after 'mete' is 'METEORITE'.
(Other definitions for meteorite that I've seen before include "Lump from space" , "Something falling from the sky" , "Piece of fallen star" , "Rock that falls to earth" , "Matter in the solar system that reaches earth" .)