One spotted in party is old minister (6)
I believe the answer is:
domino
'one spotted' is the definition.
(I've seen this before)
'in party is old minister' is the wordplay.
'in' indicates putting letters inside.
'party' becomes 'do' (do can informally mean a party).
'is old' becomes 'o' (common abbreviation eg in OE for Old English).
'minister' becomes 'min' (abbreviation).
'o'+'min'='omin'
'do' enclosing 'omin' is 'DOMINO'.
(Other definitions for domino that I've seen before include "Black and white game piece" , "Officers' mess aboard" , "Loose cloak; game piece" , "garment" , "Spotted playing tile" .)