Writer's special name for rock once (6)
I believe the answer is:
sayers
'writer's' is the definition.
(I've seen this before)
'special name for rock once' is the wordplay.
'special' becomes 'S' (common abbreviation as in Special Air Service).
'name for rock once' becomes 'Ayers' (Ayers Rock in Australia now Uluru).
's'+'ayers'='SAYERS'
(Other definitions for sayers that I've seen before include "They claim" , "Dorothy L. --, novelist" , "they announce" , "Detective fiction author, d. 1957" , "Dorothy L. ..., British crime writer" .)