One doctor left; again returned after final consideration, faster (7)
I believe the answer is:
nimbler
'final consideration faster' is the definition.
I can't tell whether this definition defines the answer.
'one doctor left again returned' is the wordplay.
'one doctor' becomes 'nimb' (I can't explain this - if you can you should believe this answer much more).
'left' becomes 'l' (common abbreviation).
'again' becomes 're' (prefix meaning 'again').
'returned' shows that the letters should be reversed in order.
're' in reverse letter order is 'er'.
'nimb'+'l'+'er'='NIMBLER'
'after' is the link.
Can you help me to learn more?
(Other definitions for nimbler that I've seen before include "Lighter moving" , "More agile" , "more alert" .)