Abusing dog whistle? (7)
I believe the answer is:
Here is my best explanation:
'abusing' is the definition.
(cursing is a kind of abusing)
'dog whistle?' is the subsidiary indication.
'dog' becomes 'cur' (cur is a kind of dog).
'whistle?' becomes 'sing' ('sing' can be a synonym of 'whistle').
(Another definition for cursing that I've seen is "Using bad language".)