Main force dash into outskirts of Beirut (5)
I believe the answer is:
brunt
'main force' is the definition.
(I've seen this in another clue)
'dash into outskirts of beirut' is the wordplay.
'dash' becomes 'run' (I've seen this before).
'into' means one lot of letters goes inside another.
'outskirts of' means to remove the middle letters.
'beirut' with its middle removed is 'bt'.
'run' placed inside 'bt' is 'BRUNT'.
(Other definitions for brunt that I've seen before include "Major force" , "Chief impact (of something bad) (bear the -)" , "Principal strength" , "The chief stress or strain" , "Burden" .)