Most remorseful about shaking (6)
I believe the answer is:
tremor
'shaking' is the definition.
(synonyms)
'most remorseful about' is the wordplay.
'about' indicates a hidden word (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)" .)