In retirement, doctor has regular sleep and runs to get fitter (7)
I believe the answer is:
nimbler
'in retirement doctor has' is the definition.
I can't judge whether this definition defines the answer.
'regular sleep and runs to get fitter' is the wordplay.
'regular' means one should take alternating letters.
'and' means one lot of letters go next to another.
'runs' becomes 'r' (cricket abbreviation).
'to' means one lot of letters go next to another (I've seen this in other clues).
'get fitter' becomes 'nimb' (I can't justify this - if you can you should believe this answer much more).
The alternating letters of 'sleep' are 'le'.
'le' put next to 'r' is 'ler'.
'ler' after 'nimb' is 'NIMBLER'.
Can you help me to learn more?
(Other definitions for nimbler that I've seen before include "more alert" , "Lighter moving" , "More agile" .)