Rubbish detective following case of Moriarty (2,3)
I believe the answer is:
my eye
'rubbish' is the definition.
(I've seen this before)
'detective following case of moriarty' is the wordplay.
'detective' becomes 'eye' (as in a private eye).
'following' is a charade indicator (letters next to each other).
'case of' means to remove the middle letters (outsides of).
'moriarty' with its centre removed is 'my'.
'eye' put after 'my' is 'MY EYE'.
(Other definitions for my eye that I've seen before include "bunk?" , "Bull" , "expression of disagreement" , "Certainly not" , "Nonsense! (slang)" .)