Soft fabric to catch one fish (6)
I believe the answer is:
plaice
'fish' is the definition.
(I know that plaice is a type of fish)
'soft fabric to catch one' is the wordplay.
'soft' becomes 'p' (musical abbreviation for 'piano', soft).
'fabric' becomes 'lace' (lace is a kind of fabric).
'to catch' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'lace' enclosing 'i' is 'laice'.
'p'+'laice'='PLAICE'
(Other definitions for plaice that I've seen before include "A fish; sounds like put" , "Flatfish, valued as food" , "Important food fish" , "Position the fish, we hear" , "N. Atlantic flatfish" .)