Music shop with yard in a mess (8)
I believe the answer is:
rhapsody
'music' is the definition.
(I've seen this before)
'shop with yard in a mess' is the wordplay.
'with' is a charade indicator (letters next to each other).
'in a mess' indicates anagramming the letters.
'shop'+'yard'='shopyard'
'shopyard' with letters rearranged gives 'RHAPSODY'.
(Other definitions for rhapsody that I've seen before include "Expression of ecstatic enthusiasm" , "Ecstatic piece" , "Ecstatic written or spoken statement" , "'''... in Blue'' (Gershwin) (8)'" , "Piece of writing" .)