Job to admit one put forward (5)
I believe the answer is:
posit
'put forward' is the definition.
(thesaurus)
'job to admit one' is the wordplay.
'job' becomes 'post' (post is a kind of job).
'to admit' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'post' enclosing 'i' is 'POSIT'.
(Other definitions for posit that I've seen before include "Suppose" , "Put forward (an idea)" , "state" , "Suggest" , "advance" .)