Most remorseful about shudder (6)
I believe the answer is:
tremor
'shudder' is the definition.
(I know that shudder can be written as tremor)
'most remorseful about' is the wordplay.
'about' says the answer is hidden in the clue (the clue goes around or about the answer).
'TREMOR' is hidden in the letters of 'most remorseful'.
(Other definitions for tremor that I've seen before include "Small quake" , "feeling of excitement" , "Involuntary quiver" , "Small earthquake" , "Quivering" .)