Group of soldiers left upset about nothing (5)
I believe the answer is:
troop
'group of soldiers' is the definition.
(I've seen this before)
'left upset about nothing' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'upset' shows that the letters should be reversed in order.
'about' is an insertion indicator.
'nothing' becomes 'o' (looks like zero - 0).
'port' in reverse letter order is 'trop'.
'trop' placed around 'o' is 'TROOP'.
(Other definitions for troop that I've seen before include "Flock, large group" , "Military group" , "Gang" , "Group of people or animals" , "Parade" .)