Against main force becoming split (4)
I believe the answer is:
torn
'split' is the definition.
(thesaurus)
'against main force becoming' is the wordplay.
'against' is a charade indicator (letters next to each other).
'main force' becomes 'rn' (abbreviation for Royal Navy, Main is a sea).
'becoming' becomes 'to' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'rn' put after 'to' is 'TORN'.
(Other definitions for torn that I've seen before include "Having rips" , "Ragged" , "Divided" , "Undecided between two" , "Removed forcibly" .)