Suggest I should fill job (5)
I believe the answer is:
posit
'suggest' is the definition.
(thesaurus)
'i should fill job' is the wordplay.
'should fill' means one lot of letters goes inside another.
'job' becomes 'post' (post is a kind of job).
'i' going inside 'post' is 'POSIT'.
(Other definitions for posit that I've seen before include "Suppose" , "Dispose" , "Put forward (something) as a given" , "Put forward (a theory)" , "Suggest" .)