One confirming truth about Scotland's number one poet (9)
I believe the answer is:
versifier
'poet' is the definition.
(synonyms)
'one confirming truth about scotland's number one' is the wordplay.
'one confirming truth' becomes 'verifier' (someone who verifies something is true).
'about' is an insertion indicator.
'scotland's number one' becomes 's' (1st letter of 'scotland').
'verifier' going around 's' is 'VERSIFIER'.
(Other definitions for versifier that I've seen before include "One can produce short poem" , "linesman" , "Lay expert" , "Poet" .)