Willing? That's nice (9)
I believe the answer is:
agreeable
I believe this clue is a double definition.
'willing?' is the first definition.
('agreeable' can be a synonym of 'willing')
'nice' is the second definition.
(I've seen this before)
'that's' is the link.
(Other definitions for agreeable that I've seen before include "Cordial" , "sympathetic" , "In harmony; pleasant" , "Pleasant, pleasing" , "delightful" .)