Endlessly, Arsene manages to be more reasonable (5)
I believe the answer is:
saner
'be more reasonable' is the definition.
('sane' can be similar in meaning to 'reasonable')
'endlessly arsene manages' is the wordplay.
'endlessly' means to remove the last letter.
'manages' indicates anagramming the letters (I've seen 'managed' mean this).
'arsene' with its final letter removed is 'arsen'.
'arsen' with letters rearranged gives 'SANER'.
'to' is the link.
(Other definitions for saner that I've seen before include "Less mad" , "less as hatter?" .)