Regret insult that's spoken: nonsense! (7)
I believe the answer is:
rhubarb
'nonsense' is the definition.
(rhubarb can mean nonsense or rubbish)
'regret insult that's spoken' is the wordplay.
'regret' becomes 'rue' (synonyms).
'insult' becomes 'barb' (I've seen this before).
'that's spoken' indicates a 'sounds like' (homophone) clue.
'rue'+'barb'='ruebarb'
'ruebarb' sounds like 'RHUBARB'.
(Other definitions for rhubarb that I've seen before include "Leaf-stalks eaten with crumble" , "Word repeated by actors to simulate conversation" , "Edible purple sticks" , "confused mutter" , "Sour sticks eaten as fruit after cooking" .)