PM following no ordinary plan, ultimately (9)
I believe the answer is:
afternoon
'pm' is the definition.
(as in 4pm)
'following no ordinary plan ultimately' is the wordplay.
'following' becomes 'after' (following can mean after or as a result of).
'ordinary' becomes 'O' (abbreviation e.g. O-Levels).
'ultimately' indicates one should take the final letters.
The final letter of 'plan' is 'n'.
'after'+'no'+'o'+'n'='AFTERNOON'
(Other definitions for afternoon that I've seen before include "one not far out" , "Time of day" , "time for tea" , "several hours" , "Between noon and evening" .)