Mostly, then, I'm confused and in a dither (2,3,4)
I believe the answer is:
in the main
'mostly' is the definition.
(both can mean generally)
'then i'm confused and in a dither' is the wordplay.
'confused' indicates anagramming the letters.
'and' is a charade indicator (letters next to each other).
'dither' indicates anagramming the letters (I've seen 'dithering' mean this).
'then'+'im'='thenim'
'thenim' with letters rearranged gives 'inthem'.
'in'+'a'='ina'
'ina' with letters rearranged gives 'ain'.
'inthem'+'ain'='IN THE MAIN'
(Other definitions for in the main that I've seen before include "Frankfurters may be dipped here" , "For the large part" , "seaborne?" , "All things considered" , "where to find water" .)