Mostly hit-men organised in a group (2,3,4)
I believe the answer is:
in the main
'mostly' is the definition.
(I've seen this before)
'hit-men organised in a group' is the wordplay.
'organised' indicates anagramming the letters.
'in a group' becomes 'ain' (I am not sure about this - if you are sure you should believe this answer much more).
'hit'+'men'='hitmen'
'hitmen' anagrammed gives 'inthem'.
'inthem'+'ain'='IN THE MAIN'
Can you help me to learn more?
(Other definitions for in the main that I've seen before include "Where schools are found" , "Generally - at sea?" , "seaborne?" , "All things considered" , "Where those who walked the plank finished" .)