Endless mechanical repetition's adult pattern of work (4)
I believe the answer is:
rota
'pattern of work' is the definition.
'endless mechanical repetition's adult' is the wordplay.
'endless' means to remove the last letter.
'mechanical repetition' becomes 'rote' (I've seen this before).
'adult' becomes 'a'.
'rote' with its final letter taken off is 'rot'.
'rot'+'a'='ROTA'
(Other definitions for rota that I've seen before include "Ecclesiastical tribunal" , "Work roster" , "Duty schedule" , "Work schedule" , "Cycle of duty" .)