Mostly, then, I’m bewildered and in a stew (2,3,4)
I believe the answer is:
in the main
'mostly' is the definition.
(I've seen this before)
'then i'm bewildered and in a stew' is the wordplay.
'bewildered' indicates anagramming the letters.
'and' means one lot of letters go next to another.
'stew' indicates an anagram.
'then'+'im'='thenim'
'thenim' with letters rearranged gives 'inthem'.
'in'+'a'='ina'
'ina' anagrammed gives 'ain'.
'inthem'+'ain'='IN THE MAIN'
(Other definitions for in the main that I've seen before include "usually" , "For the large part" , "mostly" , "Almost entirely" , "Where ships are" .)