Command doctor to check king's file before operation's conclusion (5)
I believe the answer is:
reign
'command' is the definition.
(thesaurus)
'king's file before operation's conclusion' is the wordplay.
'king's file' becomes 'reig' (I can't explain this - if you can you should believe this answer much more).
'before' says to put letters next to each other.
'conclusion' says to take the final letters.
The final letter of 'operation' is 'n'.
'reig'+'n'='REIGN'
'doctor to check' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for reign that I've seen before include "Occupy the throne" , "Monarch's time in the job" , "Spell in office" , "Be champion" , "Time on throne" .)