Rotten fish is served up (3)
I believe the answer is:
bad
'rotten' is the definition.
(I've seen this before)
'fish is served up' is the wordplay.
'fish' becomes 'dab' (I've seen this before).
'is served up' says the letters should be written in reverse.
'dab' in reverse letter order is 'BAD'.
(Other definitions for bad that I've seen before include "Spoiled or evil" , "Mouldy, off" , "(Of food) decayed" , "Of poor quality" , "Ill" .)