concerned with needing rest, and left (7)
I believe the answer is:
retired
'left' is the definition.
(I've seen this before)
'concerned with needing rest' is the wordplay.
'concerned with' becomes 're' (re can mean regarding or about).
'needing' is a charade indicator (letters next to each other).
'rest' becomes 'tired' (I can't explain this - if you can you should give a lot more credence to this answer).
're'+'tired'='RETIRED'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for retired that I've seen before include "Left" , "Finished work" , "Went to bed or gave up work" , "Turned in" , "Stopped working" .)