Creature occasionally in rut? Time to stop decline (4)
I believe the answer is:
stag
'creature occasionally in rut?' is the definition.
(rut can mean a mating period)
'time to stop decline' is the wordplay.
'time' becomes 't' (abbreviation).
'to stop' is an insertion indicator (stopping can mean blocking a gap).
'decline' becomes 'sag' (I've seen this before).
't' placed inside 'sag' is 'STAG'.
(Other definitions for stag that I've seen before include "Antlered beast" , "Male animal" , "Male at party?" , "Hart" , "Male beast" .)