Writer of Rocket Man, did you say? (9)
I believe the answer is:
stevenson
'writer' is the definition.
(novelist Robert Louis Stevenson)
'rocket man did you say?' is the wordplay.
'rocket man' becomes 'Stephenson' (George Stephenson, designer of Stephenson's Rocket).
'did you say?' shows a homophone (sound like).
'stephenson' sounds like 'STEVENSON'.
'of' acts as a link.
(Other definitions for stevenson that I've seen before include "Writer of Treasure Island, d. 1894" , "R L -, Kidnapped author" , "Robert Louis --, Scottish novelist" , "Robert Louis -, Scots novelist" , "Scottish writer" .)