The most unpleasant, upsetting row with Saint (5)
I believe the answer is:
worst
'the most unpleasant' is the definition.
(I've seen this in another clue)
'upsetting row with saint' is the wordplay.
'upsetting' is a reversal indicator.
'with' means one lot of letters go next to another.
'saint' becomes 'st' (abbreviation).
'row' back-to-front is 'wor'.
'wor'+'st'='WORST'
(Other definitions for worst that I've seen before include "Direst" , "Of the poorest quality" , "Most severe" , "Least competent" , "Best; least good" .)