Legal document picked up concerning author (6)
I believe the answer is:
writer
'author' is the definition.
(I know that author can be written as writer)
'legal document picked up concerning' is the wordplay.
'legal document' becomes 'writ' (document from a court).
'picked up' is a reversal indicator.
'concerning' becomes 're' (both can mean on a particular topic).
're' in reverse letter order is 'er'.
'writ'+'er'='WRITER'
(Other definitions for writer that I've seen before include "Author; correspondent" , "Quiller" , "Scribe, author" , "See 1, 12, 15, 23 across and 3, 6, 9 down" , "See 4" .)