Most remorseful about shake (6)
I believe the answer is:
tremor
'shake' is the definition.
(I know that tremor is a kind of shake)
'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 "Quaver" , "Involuntary quiver" , "A slight shake, body or earth" , "feeling of excitement" , "Vibration from earthquake" .)