Tremble, as weakened at heart (5)
I believe the answer is:
quake
'tremble' is the definition.
(I've seen this before)
'as weakened at heart' is the wordplay.
'as' becomes 'qua' (I've seen this before).
'at heart' indicates the central letters.
The centre of 'weakened' is 'ke'.
'qua'+'ke'='QUAKE'
(Other definitions for quake that I've seen before include "Tremble (in one's boots?)" , "Tremble (with fear)" , "Tremble, shake" , "Shudder with fear" , "Vibration of the earth" .)