A little officer — that's disappointing! (3,4,3)
I believe the answer is:
not much cop
'disappointing' is the definition.
I know nothing about this answer so I can't tell whether this works.
'a little officer' is the wordplay.
'a little' becomes 'notmuch' (I've seen this before).
'officer' becomes 'cop' (cop is a kind of officer).
'notmuch'+'cop'='NOT MUCH COP'
'that's' is the link.
Can you help me to learn more?
(Other definitions for not much cop that I've seen before include "of little value" , "complaint about lack of policing" , "Feeble" , "pretty useless" , "Undersized entrant to police force" .)