Military unit is one of four in southern front that's shelled (8)
I believe the answer is:
squadron
'military unit' is the definition.
(I know that squadron is a type of air unit)
'one of four in southern front that's shelled' is the wordplay.
'one of four' becomes 'quad' (short for quadruplet, one of four children).
'in' means one lot of letters goes inside another.
'southern' becomes 's' (abbreviation).
'that's shelled' indicates the central letters (the outer letters or shell are removed).
The middle letters of 'front' are 'ron'.
's'+'ron'='sron'
'quad' put within 'sron' is 'SQUADRON'.
'is' is the link.
(Other definitions for squadron that I've seen before include "Air force grouping" , "Unit of the armed forces" , "Cavalry unit" , "Group of planes under one command" , "detachment" .)