Most irate, as monarch that is in distress (8)
I believe the answer is:
angriest
'most irate' is the definition.
('angry' can be similar in meaning to 'irate')
'monarch that is in distress' is the wordplay.
'monarch' becomes 'R' (abbreviation of Latin rex).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'in' indicates putting letters inside.
'distress' becomes 'angst' (I've seen this in another clue).
'r'+'ie'='rie'
'rie' placed within 'angst' is 'ANGRIEST'.
'as' acts as a link.
(Other definitions for angriest that I've seen before include "Most annoyed or hostile" , "Most characteristic of 1950s young men" , "Extremely mad" , "Most irate and furious" , "Most wrathful" .)