With more reason, one enters far too confused a state (1,8)
I believe the answer is:
a fortiori
'with more reason' is the definition.
('a fortiori' can be a synonym of 'with more reason')
'one enters far too confused a state' is the wordplay.
'one' becomes 'i' (Roman numeral).
'enters' is an insertion indicator.
'confused' is an anagram indicator.
'a state' becomes 'ri' (Rhode Island abbreviation).
'far'+'too'='fartoo'
'fartoo' anagrammed gives 'aforto'.
'i' placed into 'aforto' is 'afortio'.
'afortio'+'ri'='A FORTIORI'
(Other definitions for a fortiori that I've seen before include "more conclusively" , "For stronger reasons" , "with better grounds" , "By inference, even more" , "stronger premises" .)