Deep regret about short mouthful (7)
I believe the answer is:
remorse
'deep regret' is the definition.
(I've seen this before)
'about short mouthful' is the wordplay.
'about' becomes 're' (regarding**).
'short' means to remove the last letter.
'mouthful' becomes 'morsel' (morsel is a kind of mouthful).
'morsel' with its last letter removed is 'morse'.
're'+'morse'='REMORSE'
(Other definitions for remorse that I've seen before include "Guilt for a wrong deed" , "it may bite" , "Guilty conscience" , "Sorrow for wrongs committed" , "Sorrow for wrong committed" .)