Tiny policeman is ineffective (3,4,3)
I believe the answer is:
not much cop
'ineffective' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'tiny policeman' is the wordplay.
'tiny' becomes 'notmuch' (I can't justify this - if you can you should believe this answer much more).
'policeman' becomes 'cop' (slang term).
'notmuch'+'cop'='NOT MUCH COP'
'is' is the link.
Can you help me to learn more?
(Other definitions for not much cop that I've seen before include "complaint about lack of policing" , "Undersized entrant to police force" , "pretty useless" , "Feeble" , "of little value" .)