Fish up what's faulty (3)
I believe the answer is:
bad
'faulty' is the definition.
(thesaurus)
'fish up' is the wordplay.
'fish' becomes 'dab' (I've seen this before).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'dab' reversed gives 'BAD'.
'what's' acts as a link.
(Other definitions for bad that I've seen before include "Ill" , "Rotten, evil" , "Severe; of low standard" , "Substandard" , "Of poor quality" .)