Harm caused by eg a crazy reversal (6)
I believe the answer is:
damage
'harm' is the definition.
('damage' can be a synonym of 'harm')
'eg a crazy reversal' is the wordplay.
'crazy' becomes 'mad' ('mad' can be a synonym of 'crazy').
'reversal' is a reversal indicator.
'eg'+'a'+'mad'='egamad'
'egamad' in reverse letter order is 'DAMAGE'.
'caused by' acts as a link.
(Other definitions for damage that I've seen before include "vandalism?" , "'Deface, impair (6)'" , "Mar" , "Injure, spoil" , "Harm, injury" .)