Not all prawns are fresh (3)
I believe the answer is:
raw
'fresh' is the definition.
(thesaurus)
'not all prawns are' is the wordplay.
'not all' indicates a hidden word.
'RAW' is hidden in the letters of 'prawns are'.
(Other definitions for raw that I've seen before include "Not cooked" , "Callow" , "Bleeding" , "Uncooked or unsavoury" , "Untried (of recruit) - uncooked (of meat)" .)