Going to drop one clanger (4)
I believe the answer is:
gong
'clanger' is the definition.
'going to drop one' is the wordplay.
'to drop' is a deletion indicator.
'one' becomes 'i' (Roman numeral).
'going' with 'i' taken away is 'GONG'.
(Other definitions for gong that I've seen before include "Medal, award or honour (inf)" , "Dinner bell or slang medal" , "Sound it, for dinner say" , "Beat this" , "Percussion instrument - medal (slang)" .)