50 with trendy old floor covering (4)
I believe the answer is:
lino
'floor covering' is the definition.
(short for linoleum)
'50 with trendy old' is the wordplay.
'50' becomes 'l' (Roman numeral).
'with' says to put letters next to each other.
'trendy' becomes 'in' ('in' can mean fashionable).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'l'+'in'+'o'='LINO'
(Other definitions for lino that I've seen before include "Floor covering (abbr)" , "put this down?" , "'Floor covering, in short (4)'" , "Tough floor covering" .)