For one promoted, Horace's work rocks (6)
I believe the answer is:
geodes
'rocks' is the definition.
'for one promoted horace's work' is the wordplay.
'for one' becomes 'eg' (I've seen this in other clues).
'promoted' is a reversal indicator (going up, in a down clue).
'horace's work' becomes 'odes' (Odes is a work by Roman poet Horace).
'eg' reversed gives 'ge'.
'ge'+'odes'='GEODES'
(Other definitions for geodes that I've seen before include "Stones" , "Rock cavities lined with crystals - see dog (anag)" .)