Show mercy to viper coiled in tree, heading off (8)
I believe the answer is:
reprieve
'show mercy to' is the definition.
(to reprieve someone is to show them mercy)
'viper coiled in tree heading off' is the wordplay.
'coiled' indicates an anagram (letters coiled up).
'in' means one lot of letters goes inside another.
'heading off' means to remove the first letter.
'viper' is an anagram of 'priev'.
'tree' with its first letter taken away is 'ree'.
'priev' going inside 'ree' is 'REPRIEVE'.
(Other definitions for reprieve that I've seen before include "Suspend a death sentence." , "Respite - stay of execution" , "Postponement of punishment" , "Forgive" , "Temporary relief, like stay on death sentence" .)