Acceptable answer about concerning waste product (4)
I believe the answer is:
urea
'waste product' is the definition.
(compound in urine)
'acceptable answer about concerning' is the wordplay.
'acceptable' becomes 'u' (as in U and non-U speech).
'answer' becomes 'a' (abbreviation eg in Q and A).
'about' means one lot of letters go next to another.
'concerning' becomes 're' (both can mean on a particular topic).
'a' put after 're' is 'rea'.
'u'+'rea'='UREA'
(Other definitions for urea that I've seen before include "It's produced by piddling" , "wee substance?" , "Waste material" , "Waste product of mammals" , "Compound found in urine, synthesised as a fertiliser" .)