Is breaking current record (4)
I believe the answer is:
disc
'record' is the definition.
(synonyms)
'is breaking current' is the wordplay.
'breaking' means one lot of letters goes inside another.
'current' becomes 'dc' (abbreviation for direct current).
'is' going into 'dc' is 'DISC'.
(Other definitions for disc that I've seen before include "'Round, flat shape (4)'" , "Gramophone record" , "Old record" , "Capital of India" , "put on by jockey?" .)