Fish -- four caught, still breathing (6)
I believe the answer is:
living
'breathing' is the definition.
(thesaurus)
'fish four caught still' is the wordplay.
'fish' becomes 'ling' (ling is a kind of fish).
'four' becomes 'iv' (roman numeral).
'caught still' means one lot of letters goes inside another (I've seen this in other clues).
'ling' enclosing 'iv' is 'LIVING'.
(Other definitions for living that I've seen before include "Quick; benefice" , "Extant" , "parish" , "sustenance" , "Existing" .)