Leader to check one in test (9)
I believe the answer is:
editorial
'leader' is the definition.
(I've seen this before)
'check one in test' is the wordplay.
'check' becomes 'edit' (I've seen this before**).
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'test' becomes 'oral' (oral is a type of examination).
'i' put into 'oral' is 'orial'.
'edit'+'orial'='EDITORIAL'
'to' acts as a link.
(Other definitions for editorial that I've seen before include "bit of paper" , "Article giving a newspaper's opinion" , "Leading article" , "considered view on paper" , "Newspaper opinion piece" .)