Rug with top-class exterior is an expensive fiddle (5)
I believe the answer is:
amati
'expensive fiddle' is the definition.
(violin created by the Amati family)
'rug with top-class exterior is an' is the wordplay.
'rug' becomes 'mat' (both are floor coverings).
'with' means one lot of letters go next to another.
'top class exterior' becomes 'a' (I can't explain this - if you can you should believe this answer much more).
'is an' becomes 'i' (Roman numeral for one).
'mat' put after 'a' is 'amat'.
'amat'+'i'='AMATI'
(Other definitions for amati that I've seen before include "Famous violin maker" , "instrument-maker" , "Great fiddle" , "fiddler" , "Italian craftsmen" .)