Most remorseful about shaking (6)
I believe the answer is:
tremor
'shaking' is the definition.
(synonyms)
'most remorseful about' is the wordplay.
'about' indicates the answer is hidden within 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 "Shiver in the earth" , "A little agitation" , "Quaver" , "Quivering" , "A small shake, perhaps of earth ( 6)" .)