Not required to be forgiving (5)
I believe the answer is:
spare
This clue is a double definition.
'not required' is the first definition.
(I've seen this before)
'to be forgiving' is the second definition.
(to spare someone is to pardon them)
(Other definitions for spare that I've seen before include "Additional; thin" , "Thin; surplus" , "With no excess fat" , "Surplus to requirements" , "Additional - for use when needed" .)