Fat monarch in pantry (6)
I believe the answer is:
larder
'pantry' is the definition.
(synonyms)
'fat monarch' is the wordplay.
'fat' becomes 'lard' (lard is a kind of fat).
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'lard'+'er'='LARDER'
'in' is the link.
(Other definitions for larder that I've seen before include "Room or cupboard for storing food" , "Food store" , "Store cupboard" , "Food storeroom" , "Buttery" .)