Regret nasty comment being heard? Nonsense! (7)
I believe the answer is:
rhubarb
'nonsense' is the definition.
(rhubarb can mean nonsense or rubbish)
'regret nasty comment being heard?' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'nasty comment' becomes 'barb' (I've seen this before).
'being heard?' shows a homophone (sound like).
'rue'+'barb'='ruebarb'
'ruebarb' sounds like 'RHUBARB'.
(Other definitions for rhubarb that I've seen before include "Large-leaved plant" , "Reddish leafstalks cooked and sweetened" , "Meaningless talk" , "Rheum rhaponticum" , "Reddish, fleshy-stalked plant, makes nice tart" .)