Tedious routine involving stags in autumn? (3)
I believe the answer is:
rut
This clue is a double definition.
'tedious routine' is the first definition.
(I've seen this before)
'involving stags in autumn?' is the second definition.
(rut is mating season for deer)
(Other definitions for rut that I've seen before include "Deep track or tedious routine" , "A worn groove - don't live in one" , "mate" , "animal passion" , "It's hard to get out of" .)