Most remorseful about shaking (6)
I believe the answer is:
tremor
'shaking' is the definition.
(I know that tremor is a type of shaking)
'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 within '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)" .)