Refuse to accept name in speech that's frowned on (5)
I believe the answer is:
slang
'speech that's frowned on' is the definition.
I can't tell whether this definition defines the answer.
'refuse to accept name' is the wordplay.
'refuse' becomes 'slag' (type of industrial waste matter).
'to accept' is an insertion indicator.
'name' becomes 'n' (common abbreviation e.g. on forms).
'slag' going around 'n' is 'SLANG'.
'in' is the link.
Can you help me to learn more?
(Other definitions for slang that I've seen before include "Unconventional language" , "colloquialisms" , "Insult" , "Non-standard vocabulary" , "manner of speaking" .)