Medal disappearing, one lost (4)
I believe the answer is:
gong
'medal' is the definition.
(I've seen this before)
'disappearing one lost' is the wordplay.
'disappearing' becomes 'going' (going is a kind of disappearing).
'one' becomes 'i' (Roman numeral).
'lost' is a deletion indicator.
'going' with 'i' taken out is 'GONG'.
(Other definitions for gong that I've seen before include "Round metal plate struck to resound" , "Medal, award or honour (inf)" , "Resonant metal disc" , "Wordsworth's Boy; Dostoevsky's hero" , "Kind of bell sounded for dinner" .)