Job one secured in advance (5)
I believe the answer is:
posit
'advance' is the definition.
(I've seen this before)
'job one secured' is the wordplay.
'job' becomes 'post' (post is a kind of job).
'one' becomes 'i' (Roman numeral).
'secured' indicates putting letters inside.
'post' placed around 'i' is 'POSIT'.
'in' acts as a link.
(Other definitions for posit that I've seen before include "Lay down as principle" , "Put forward (something) as a given" , "advance" , "Put forward; place" , "forward-looking statement" .)