Mostly sickly, suffering, beset by palpitation - mercury? (11)
I believe the answer is:
quicksilver
'mercury?' is the definition.
('quicksilver' can be a synonym of 'mercury')
'mostly sickly suffering beset by palpitation' is the wordplay.
'mostly' means to remove the last letter.
'suffering' indicates anagramming the letters.
'beset by' is an insertion indicator ('beset' could historically mean 'cover').
'palpitation' becomes 'quiver' ('quiver' can be a synonym of 'palpitation').
'sickly' with its final letter taken away is 'sickl'.
'sickl' with letters rearranged gives 'cksil'.
'cksil' placed within 'quiver' is 'QUICKSILVER'.
(Other definitions for quicksilver that I've seen before include "Element of dentistry" , "Liquid mercury" , "Mercury - volatile" , "so rapid" .)