I weep with soldiers returning to front line (6)
I believe the answer is:
isobar
'front line' is the definition.
(I've seen this in another clue)
'i weep with soldiers returning' is the wordplay.
'weep' becomes 'sob' (sobbing is a kind of weeping).
'with' means one lot of letters go next to another.
'soldiers' becomes 'RA' (Royal Artillery).
'returning' says the letters should be written in reverse.
'ra' backwards is 'ar'.
'i'+'sob'+'ar'='ISOBAR'
'to' acts as a link.
(Other definitions for isobar that I've seen before include "A Boris (anag.)" , "depression indicator?" , "Met map pressure line" , "(Map) line of constant atmospheric pressure" , "It connects points of equal barometric pressure" .)