Fat sovereign in store (6)
I believe the answer is:
larder
'store' is the definition.
(food store)
'fat sovereign' is the wordplay.
'fat' becomes 'lard' (lard is a kind of fat).
'sovereign' becomes 'er' (Abbreviation for Elizabeth Regina).
'lard'+'er'='LARDER'
'in' acts as a link.
(Other definitions for larder that I've seen before include "Cupboard or small room for food" , "Store cupboard" , "Place where food is kept" , "Room or cupboard for food storage" , "Food storeroom" .)