Rotten fish is sent back (3)
I believe the answer is:
bad
'rotten' is the definition.
(similar in meaning)
'fish is sent back' is the wordplay.
'fish' becomes 'dab' (dab is a type of fish).
'is sent back' is a reversal indicator.
'dab' backwards is 'BAD'.
(Other definitions for bad that I've seen before include "Harmful" , "Ill" , "SECOND HALF OF 16" , "Evil, below standard or off" , "Substandard" .)