Stop ruminants stealing last of breakfast (5)
I believe the answer is:
deter
'stop' is the definition.
(to deter behaviour is to stop it)
'ruminants stealing last of breakfast' is the wordplay.
'ruminants' becomes 'deer' (deer are a type of ruminant).
'stealing' indicates putting letters inside.
'last of' says to take the final letters.
The last letter of 'breakfast' is 't'.
'deer' going around 't' is 'DETER'.
(Other definitions for deter that I've seen before include "Discourage, put off" , "Discourage (someone from an action)" , "Discourage (an activity)" , "Daunt" , "Put off (from an action)" .)