Flipping rotten fish (3)
I believe the answer is:
dab
'fish' is the definition.
(I've seen this before)
'flipping rotten' is the wordplay.
'flipping' shows that the letters should be reversed in order.
'rotten' becomes 'bad' ('bad' can be similar in meaning to 'rotten').
'bad' in reverse letter order is 'DAB'.
(Other definitions for dab that I've seen before include "Expert; flatfish" , "A bad, backward fish" , "Touch gently - fingerprint" , "Light stroke" , "Fish; small amount" .)