Fish that's terrible being served up (3)
I believe the answer is:
dab
'fish' is the definition.
(I've seen this before)
'terrible being served up' is the wordplay.
'terrible' becomes 'bad' ('bad' can be similar in meaning to 'terrible').
'being served up' says the letters should be written in reverse.
'bad' in reverse letter order is 'DAB'.
'that's' acts as a link.
(Other definitions for dab that I've seen before include "Digital radio" , "[SOMETHING FISHY]" , "Apply light touches of colour to the fish" , "A bad, backward fish" , "Expert; flat fish" .)