Mad about Queen Elizabeth's porcelain (7)
I believe the answer is:
pottery
'porcelain' is the definition.
(thesaurus)
'mad about queen elizabeth's' is the wordplay.
'mad' becomes 'potty' (informal term).
'about' is an insertion indicator.
'queen elizabeth' becomes 'ER' (abbreviation for Elizabeth Regina).
'potty' going around 'er' is 'POTTERY'.
(Other definitions for pottery that I've seen before include "Ceramics factory" , "baked goods" , "Goods of fired clay" , "Items of fired clay" , "Earthenware" .)