One of those going out a lot with a Falstaffian character from a spy ring (5,7)
I believe the answer is:
heavy sleeper
'one of those going out a lot with a falstaffian' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'character from a spy ring' is the wordplay.
'character from a spy' becomes 'heavy' (I am not sure about this - if you are sure you should believe this answer much more).
'ring' becomes 'sleeper' (I've seen this before).
'heavy'+'sleeper'='HEAVY SLEEPER'
Can you help me to learn more?
(Another definition for heavy sleeper that I've seen is " Person difficult to wake".)