Rotten fish, sent back (3)
I believe the answer is:
bad
'rotten' is the definition.
(similar in meaning)
'fish sent back' is the wordplay.
'fish' becomes 'dab' (dab is a type of fish).
'sent back' says the letters should be written in reverse.
'dab' back-to-front is 'BAD'.
(Other definitions for bad that I've seen before include "Spoiled or evil" , "SECOND HALF OF 16" , "not healthy" , "Wicked" , "Evil, below standard or off" .)