The rotten fish is sent back (3)
I believe the answer is:
bad
'the rotten' is the definition.
(I've seen this before)
'fish is sent back' is the wordplay.
'fish' becomes 'dab' (dab is a type of fish).
'is sent back' says the letters should be written backwards.
'dab' in reverse letter order is 'BAD'.
(Other definitions for bad that I've seen before include "Wicked" , "(Of food) decayed" , "Substandard" , "Mouldy, off" , "Stinking" .)