Forgiving deranged genius holding wrong key (8)
I believe the answer is:
excusing
'forgiving' is the definition.
(excusing is a kind of forgiving)
'deranged genius holding wrong key' is the wordplay.
'deranged' indicates anagramming the letters.
'holding' means one lot of letters goes inside another.
'wrong key' becomes 'xc' (I am not sure about this - if you are sure you should believe this answer much more).
'genius' with letters rearranged gives 'eusing'.
'eusing' enclosing 'xc' is 'EXCUSING'.
Can you help me to learn more?
(Other definitions for excusing that I've seen before include "Pardoning" , "Overlooking a fault" , "Letting off or granting an exemption" , "Condoning" .)