Fifty fish nets reserved (4)
I believe the answer is:
cold
'reserved' is the definition.
(thesaurus)
'fifty fish nets' is the wordplay.
'fifty' becomes 'l' (Roman numeral).
'fish' becomes 'cod' (type of fish).
'nets' indicates putting letters inside (some letters are contained or netted).
'l' put within 'cod' is 'COLD'.
(Other definitions for cold that I've seen before include "complaint" , "Chilly; minor ailment" , "Stand-offish" , "Dead" , "Mild viral infection, very common" .)