Put forward first person in job (5)
I believe the answer is:
posit
'put forward' is the definition.
(thesaurus)
'first person in job' is the wordplay.
'first person' becomes 'i'.
'in' means one lot of letters goes inside another.
'job' becomes 'post' (post is a kind of job).
'i' inserted inside 'post' is 'POSIT'.
(Other definitions for posit that I've seen before include "Put forward (something) as a given" , "advance" , "Put forward (an idea)" , "given proposition" , "forward-looking statement" .)