Job maybe available at the start – one that's clearly understood (4,4)
I believe the answer is:
open book
'one that's clearly understood' is the definition.
(an easily understood person is an open book)
'job maybe available at the start' is the wordplay.
'job maybe' becomes 'book' (I can't explain this - if you can you should believe this answer much more).
'available' becomes 'open' (similar in meaning).
'at the start' means one lot of letters go next to another.
'book' after 'open' is 'OPEN BOOK'.
Can you help me to learn more?
(Other definitions for open book that I've seen before include "Face etc that can be easily read" , "Person easily understood" , "Something that can be easily interpreted" .)