Not immediately required to show mercy (5)
I believe the answer is:
spare
'show mercy' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for spare that I've seen before include "Not required" , "Excess to requirements" , "Thin; left over" , "In reserve; pardon" , "slit at top of trousers in Scotland" .)