Scientists apparently now expect Republican leaders to be comparatively rational (5)
I believe the answer is:
saner
'be comparatively rational' is the definition.
('sane' can be similar in meaning to 'rational')
'scientists apparently now expect republican leaders' is the wordplay.
'leaders' says to take the initial letters.
The first letters of 'scientists apparently now expect republican' is 'SANER'.
'to' is the link.
(Other definitions for saner that I've seen before include "less as hatter?" , "Less mad" .)