Regular soldiers put November in events book (8)
I believe the answer is:
Here is my best explanation:
'regular' is the definition.
'soldiers put november in events book' is the subsidiary indication.
'soldiers put' becomes 'or' (I can't explain this - if you can you should give a lot more credence to this answer).
'november' becomes 'n' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' indicates putting letters inside.
'events book' becomes 'diary'.
'n' going inside 'diary' is 'dinary'.
(Other definitions for ordinary that I've seen before include "Commonplace, avergae", "Run of the mill", "Plain", "Normal or average", "Normal, mundane", "Run-of-the-mill", "Unexceptional, quite usual", "Customary", "Average" and "'Commonplace, banal (8)'".)