Against main force in Split (4)
I believe the answer is:
torn
'split' is the definition.
(I've seen this in another clue)
'against main force' is the wordplay.
'against' becomes 'to' (as in a lean-to, building leaning against anothe).
'main force' becomes 'RN' (abbreviation for Royal Navy, Main is a sea).
'to'+'rn'='TORN'
'in' acts as a link.
(Other definitions for torn that I've seen before include "Ripped - undecided" , "Removed forcibly" , "Rent, ripped" , "Divided" , "Having rips" .)