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' is an insertion indicator (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" .)