Royal command following, popular with men fit for service (2,7,5)
I believe the answer is:
in working order
'fit for service' is the definition.
'royal command following popular with men' is the wordplay.
'royal' becomes 'king' (I've seen this before).
'command' becomes 'order' (order is a kind of command).
'following' says to put letters next to each other.
'popular with' becomes 'inw' (I can't justify this - if you can you should give a lot more credence to this answer).
'men' becomes 'or' (military abbreviation for Other Ranks).
'king'+'order'='kingorder'
'inw'+'or'='inwor'
'kingorder' after 'inwor' is 'IN WORKING ORDER'.
Can you help me to learn more?
(Other definitions for in working order that I've seen before include "monk who doesn't simply pray?" , "Functioning properly" .)