Soldiers composed unfinished Mass, one for the departed (7)
I believe the answer is:
requiem
'one for the departed' is the definition.
(musical composition dedicated to the dead)
'soldiers composed unfinished mass' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'composed' becomes 'quiet'.
'unfinished' means to remove the last letter.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'quiet' with its final letter taken away is 'quie'.
're'+'quie'+'m'='REQUIEM'
(Other definitions for requiem that I've seen before include "Funeral music" , "RC mass for the dead" , "Mass for the departed" , "Funeral service" , "shark?" .)