It's not required to be merciful (5)
I believe the answer is:
spare
'it's not required' is the definition.
(I've seen this before)
'be merciful' is the wordplay.
I cannot really see how this works, but
'be' could be 'are' (I've seen this in another clue) and 'are' is found in the answer.
This explanation may well be incorrect...
'to' acts as a link.
Can you help me to learn more?
(Other definitions for spare that I've seen before include "Refrain from using" , "A feature of trousers" , "Excess to requirements" , "This tyre is for emergencies" , "Extra item just in case" .)