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 am not sure about this - if you are sure you should give a lot more credence to this answer).
'available' becomes 'open' (similar in meaning).
'at the start' is a charade indicator (letters next to each other).
'book' put 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" .)