Person checking lines succeeded as a linesman? (9)
I believe the answer is:
versifier
'a linesman?' is the definition.
(someone who writes lines of poetry)
'person checking lines succeeded' is the wordplay.
'person checking' becomes 'verifier' (someone verifying or checking things).
'lines' is an insertion indicator (line could historically mean to cover something).
'succeeded' becomes 's' (genealogical abbreviation).
'verifier' going around 's' is 'VERSIFIER'.
'as' is the link.
(Other definitions for versifier that I've seen before include "One can produce short poem" , "Poet" , "Lay expert" , "linesman" .)