Officer, before mass trouble in the rear, does my job (8)
I believe the answer is:
compiles
'does my job' is the definition.
(compiling crosswords is the crossword setter's job)
'officer before mass trouble in the rear' is the wordplay.
'officer' becomes 'co' (Commanding Officer abbreviation).
'before' means one lot of letters go next to another.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'trouble in the rear' becomes 'piles' (piles can informally mean haemorrhoids).
'co'+'m'+'piles'='COMPILES'
(Another definition for compiles that I've seen is " Composes from several sources".)